Standalone 

The 26" -> 700c adapter I ordered from Xtracycle "fits," but puts the brake shoes at the absolute bottom of their adjustment. It seems to me that this is causing less braking power. Is there any way to increase leverage?

I guess I might eventually want to do a disc rear, but that will be $$$$....

fietsbob

Pauls Components of Chico California, makes a V brake which the pad mount grips the round lever.
so will slide further down ..
I'd look at all that housing, for compression mushy-ness,
It could be part of the problem..

A hydraulic brake would be better, with all that distance between hand lever
and brake caliper.

only Magura makes a hydraulic rim brake, but they have made them for 20 years..

DCB0

With V brakes, having the shoes lower gives you more braking power, not less.

Standalone 

I was thinking that since they would be "travelling" further when slotted higher, they'd be putting more power to the wheel. But it occurs to me that this would be like saying you get more power from a larger rear cog... so obviously you're right.

I tended to the brake pads and pulled the rim back over with a little tweak here and there, and all is well. Taking a file to the pads put them right as rain. Messing around with sandpaper wasn't getting the job done, I guess.
So I take it back. The xtra brake adapter definitely fits.

FastJake

V-brakes are pretty darn powerful. They should stop you very well with proper setup and good pads. Sounds like you got it figured out though. Get some Kool Stop pads when you replace your current set and be careful not to throw yourself over the bars the first time you stop