from a cursory look at the 'net, it seems like Campy 9 & 10 speed cassettes would be interchangeable (ie, use the same hub...maybe even the same for 11?)...is this correct?

I have a full 10 speed record group I am thinking of upgrading the 9 speed Veloce/Mirage/Daytona group on one of my bikes but I will only do it any time soon if I can use the wheelset & freehub that is on the bike now as a 9 speed. (ultimately doesn't matter 'cause the mixed group will be going back on something sooner or later so I'll need another wheel at some point in the future)

thanks in advance for indulging this basic question!

Yes, you can use current campy hub for both 9 spd and 10spd cassettes. Some hubs (eg White Industries H2) work with 9 and 10 speed only but not 11 speed.

Right.

Also note that first generation (Chorus and Record from 1997/1998, everything else through 1999) 9 speed freehubs have a 26mm threaded lock ring (CS-201 12T+ first cog, CS-301 for 11T) versus 27mm for later hubs (Anything with an over-sized aluminum axle or cartridge bearings - CS-401 12T+ first cog, CS-501).

The 11T lock ring included with a 10s cassette won't mate to one of the older hubs and you'd need to find the appropriate part on E-bay.

Followup to this string. Can I mix Campy Veloce 9-speed and 10-speed cogs? I have a bad T16 on my 9-speed Campy Veloce set due to single speed use on that one for a long time, so need to replace it. I wouldn't mind also reconfiguring the mix to include an 11T (it now starts at 12T). I see a fairly cheap 10-speed Veloce cassette starting at 11T on ebay, so I'm wondering if I can use that set and just remove a cog, or use a couple cogs from it to replace on my existing set.

You'll probably find the thickness of the sprockets is slimmer as the speeds gets higher and so your shifting will start to go all wacky (and that;s if your chain doesn't skip or properly engage). For more info about the differences between gear spacing see this:
https://www.cyclinguk.org/cyclists-l...gears/shimergo
but be advised if you start going down the rabbit hole of mixing and matching sprockets and shimming and all that entails, you will encounter a lot of rabbits.

Sprockets are timed, so they only work when in the original order. If you have worn cogs, get a new cassette. The lowest level Campy cassettes are reasonably priced. Let google do the shopping.

1 swapped sprocket, with the correct spacer might be ok. If you already have everything, give it a try.