Is there an open-source, super lightweight WDC 65C02 assembler? My needs are somewhat strange: I can't run it as a standalone binary, I need to be able to simply include it as a static lib as part of another project. Due to this, I'm guessing that an open-source version can be adapted in this way. Other than that, anything in the C family should work: C, C++, hell even Objective-C. Any ideas?

Hmm, can't think of anything. Assemblers are usually self contained and operate file to file.
There are many listed in the reference section:
http://6502.org/tools/asm/

What does an assembler library look like to you? Does it take a massive text blob and produce a large binary blob? Or is it some kind of line-by-line mini-assembler, with barely a symbol table?

Aw, Ed - what about the assembler included in your beloved Beeb's BASIC? It's possible he's thinking along those lines!

A giant text blob would be ideal. It can operate on files if necessary, it just would be a library rather than a forked, standalone process.

It looks as if ca65 from the cc65 suite would do what you need. The input abstraction is a CharSource with a simple set of functions:

So basically you'd need to write a driver for this that sets that up and calls the assembler, similar to how it's done in main():

(If you follow down through InitScanner you'll see how it sets up the CharSource.) Then you just link your driver against those files and you should be set!

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Curt J. Sampson - github.com/0cjs

Good find Curt!

Indeed, using the 16k ROM of BBC Basic would be a way, perhaps with lib6502 to emulate it. A 64k-sized emulation of a 6502 machine running a Basic interpreter isn't all that large in terms of modern software - that's not a point which naturally occurs to me! Lib6502 is written to be an emulation library, and is plain C and freely reusable.
https://www.piumarta.com/software/lib6502/

An emulation library isn't quite the same as an assembler library, so Curt's answer is surely the better approach.

Yes I don't need an emulation library, I built my own (though I did refer to lib6502 often in my implementation) I just need an in-process assembler. Looking at ca65 but I haven't done C in quite awhile and it's taking me a bit of time adapting it. Thank you for suggestions, though! Will update with progress at some point, I hope.

I wrote a fairly simple assembler (no macros) as part of the C02 project. Right now it reads and writes to files, but it should be simple enough to adapt it to use text and binary blobs. The files are a02.c and a02.h at https://github.com/RevCurtisP/C02.