The Flex SDK contains compiled libraries (/frameworks/libs/*.swc files) for all framework classes.
The SWCs (zip archives) each contain a "library.swf" generated by the mxmlc compiler.
The library.swf likely contains a single tag containing "abc" bytecode, including a constant pool (which is what you're after).

I haven't found a parser implemented in Java, but there is one written in OCaml, part of the "haxe" language/compiler:
http://haxe.org/tutos/flash9?s=flash#as3_hx_header_generation
http://haxe.org/tutos/flex_and_haxe_intro#creating_and_calling_as3_classes_from_haxe

http://cvs.motion-twin.com/horde/chora/co.php?rt=haxe&r=1.43&f=genswf9.ml
http://cvs.motion-twin.com/horde/chora/browse.php?rt=ocaml&f=%2Fswflib%2F

Tamarin project also contains relevant extraction tools: http://www.5etdemi.com/blog/archives/2007/01/as3-decompiler/

I know that the classes could be decompiled from swc/swfs but the license seems to preclude one from doing this :)

Maxim, which specific license are you referring to? The Flex SDK license, or a different one?

Maxim, can you provide a link to the license you were referring to?

A tool that just scans constant pools to extract class/method signatures would be allowed for interoperability reasons, I would assume.

http://www.adobe.com/licensing/developer/fileformat/license/

a. You may not use the Specification in any way to create or develop a runtime, client, player, executable or other program that reads or renders .swf files.

b. You may not use the Specification in any way to create a server, executable, or other program that will stream or deliver data from a client to a server, from a server to another server, or from a server to a client.

c. You will not make or distribute copies of the Specification, or electronically transfer the Specification outside your company.

Maxim,

That license only applies if you want to obtain the official file format specification for SWF9 from Macromedia.

But there is no need to do that, there is enough information available in public to build a constant pool reader.
Especially with the opensource donation of Tamarin project, which actually includes a full disassembler.

Maxim, what kind of solution did you implement?

Java swf / abc decoder (based on abcdump.as from Tamarin sources) that dumps as files reflecting class / members information