Altering music
Is there no way to select / use your own music via the puzzle creator?
If so, can this be done via Hammer and still submitted to the workshop?
I don't think puzzle creator will let you do it, Hammer certainly does. I am pretty sure you can submit Hammer'd maps to the workshop as long as it's all packed as one file.
This.
You can package any custom content into your .bsp using pakrat and publish it through the p2map_publish.exe located in Portal 2/bin. Make sure you put in the entities needed in order for it to work in the workshop. See Mevious' tutorial video post: puzzle-creator/hammer-tutorial-advanced-workshop-integration-for-portal-2-t6136.html
Scripts, models, sounds, textures etc. can be added into your .bsp but make sure you get someone to download it (publish it to friends) and test it before you publish it for real.
Awesome, thanks. Also, does it support the creation of co-op maps?
PeTI? No. Hammer? Definately.
Hammer is the editor used internally by Valve. Pretty much anything that is done in the game can be done in Hammer.
As for the workshop, I can't be certain but I'm pretty sure there is no support for Co-op maps at this stage but I'm sure it will be here eventually.
Oh, thanks. Is there any confirmation from Valve about their intentions to expand upon the capabilities of the PeTI like that?
Official confirmation? Not that I know of, but that isn't unusual.
Valve doesn't like to talk about what they have in the works until it is near completion (exceptions being DoTA2 and CS:GO because they need community feedback for those products). I am almost certain co-op support will be added eventually.
That would be pretty cool. The PeTI is an awesomely intuitive piece of kit, but way too limited as it is.