steal working code/ideas from OGL:Sandbox into PDL's TriD, and textures is an obvious candidate. Are there others?

The single most awesome bit of code in OpenGL::Sandbox is $program->set_uniform. The documentation doesn't do it justice, but it performs magic that lets you set the uniform values for the shader in a very convenient manner. There might be an interesting way to tie that into PDL in a passive manner so that the user specifies everything in terms of PDL arrays, and PDL magically broadcasts into the graphics card for the final step instead of broadcasting across CPUs. (that's what Uniforms are, basically; global variables that get broadcast to N texture units each running a copy of the shader, and each with shared access to any of the texture or buffer objects that you have loaded.)

Feel free to steal the code. I feel like it hasn't received its return-on-investment for the number of hours I put into it.

For textures, you need to weigh the convenience vs. adding a dependency on PNG and JPEG libraries. But, you can always fall back to raw uncompressed RGBA files.