I got an 86 Peugeot Triathlon, and it uses a quill seat post. I've been riding the bike, and I'd like to tilt the nose of the seat down a little. This is with a Brooks B17 saddle (which I have not used before). I'd like to use a seatpost with an angle adjustment, but cannot find one. In fact, the only quill seatpost I've found is a Campy, which doesn't have an angle adjustment either.
Is there a way to adapt to a clamped seatpost?

Do a search for a quill post that BF member Grand Bois made from a conventional seatpost.

Have a machine shop modify a plain seat post ,
by tapering the inside of the bottom
of what seat post fits, and then threading
and making a matching tapered plug
that will expand the seat post inside the frame.

Plain post is useful because the bolt head and a flat washer,
spanning across the top will be fine.

then you buy a nice saddle clip for plain seat-posts ..
I like the one Brompton makes , step less and light, 1 bolt.

Big Brooks seller Wallingford bike carries them, as does any Brompton dealer.

Pardon my ignorance, but I don't understand your post (no pun), and despite 40+ uears in the bike industry, I've never seen or heard of a seatpost that doesn't allow saddle angle adjustment.

By quill post do you mean one which expands at the bottom, rather than depending on a frame pinch bolt as most do?

If so, most of these are of the plain pin type, and take saddle clamp assembly at the top. Saddle clamps allow almost 360° of saddle adjustment, from almost vertical, nose down to the same, nose up.

If OTOH you mean that your current post has an integrated saddle clamp, but the range of adjustment is too limited, then your option is to buy a plain pin post and clamp, or a different I-clamp post such as the old Onza post which allow for a larger range of adjustment than most. (however may not be available in the right OD for your frame.

If I haven't addressed your problem, please clarify exactly what it is, and I or others may be able to point you in the right direction.

I did a google search for 'quill seatpost' and every single one, without exception, that I found had either a built-in adjustable clamp or a reduced diameter at the top where a basic seatpost clamp (also asdustable) could go. Do you have a picture of the seapost in question as I cannot imagine a seatpost that does not allow any angle adjustments.

Like Well Biked said, there are also lots of examples of people modifying standard seatposts to use an expander or wedge.

Edit: FBinNY beat me to it with a comment about all seatposts having saddle angle adjustability. However! Some kids bikes (and maybe some ultra cheap dept. store bikes, but I cannot recall any examples) have a saddle permanently mounted on top of a straight post with no adjustability.

Quill seatposts work like a quill stem in regard to what holds them in place. The bikes I've seen them on are late '80's/perhaps early '90's aluminum framed Schwinns, and some steel framed Peugeot's from the late eighties. I've never seen one that didn't have problems; either broken completely, cracked, etc. One of the dumbest ideas ever IMO. But there are defiinitely bikes out there that were designed for these posts.

Maybe Grand Bois will see this thread and post pics of the post he made from a regular post. I remember being impressed by it.

I think FB knows and has seen many examples of what is commonly referred to as a 'quill seatpost,' but, like me, has never seen one without the ability to adjust saddle angle.

I'm sure you're right.

Once I took the saddle off, I can see that it does have angle adjustment. The bottom block for the seat rails slides along the downward curved top of the post giving angle adjustment with some forward movement of the seat.

The way this one is designed varies from the angled plug I see on quill stems. On this one, the plug is a round taper and expands a split sleeve against the tapered end of the post such that the sleeve expands evenly. It came out dry, so I need to grease it up and put it back.

There are several quill stems that are designed with expander plugs like the one on your seatpost so it isn't a unique item. The angle adjustment saddle clamp is also a common type found on lower-line and some mid-line seatposts and usually referred to as a Laprade type.

It's a minor PITA to get this one at the right angle and rail position, not to mention adjusting height with the seat on. This also adds a lot of unnecessary weight for the sake of eliminating a seatpost collar. Oh well, I guess I can live with it.

Well, nobody said it was a good design, just one you are stuck with unless.....

Does your frame have at least 1/2" of seat tube projecting above the toptube? If so, you could drill a small (~3/16") hole about 1" below the top of the seat tube, cut a slot from the top down to the hole and then add a suitable diameter seatpost collar. That would open up an entire range of seatpost and saddle options.

It barely sticks up 1/4" toward the front, flush at the back, so there's nothing to mount a collar on. Since for me the seatpost is set it and forget it, it should be OK.