I'm specifically interested in using my phone (Samsung Galaxy S5) to work on software for my Commodore PET 2001.

There are a few Commodore emulators out there, including VICE. I have a git client already, and I am pretty sure I could get the xa65 cross assembler to run on Android, and the DroidEdit text editor is good. I carry the phone around in my pocket all the time, but the PET, not so much. It would be great to work on 6502 code when I'm waiting in line at Starbucks or riding the Metro or whatever. Is anybody doing this already?

It's possible that http://www.asm80.com/ would be useful to you: it's open source so if it's got missing features you could conceivably add them (for example it emulates KIM-1, but not PET)

For little snippets of code (which you'd paste in from an editor like DroidEdit) you might use easy6502. Again, it doesn't presently emulate a real machine, but it could.

I would be very interested to hear if you got xa65 to cooperate with Android. I just finished the 2.3.7 update and I would be willing to accept patches for Android (you'll probably need a Dalvik shell around the xa core).

I got it working today, in the 'Termux' app on a Samsung Galaxy S7!

Where 'it' is a pronoun which here has an antecedent of:
Linux command line (provided by Termux)
gcc
make
xa65
ruby
git
ssh (client only -- I'm not rooted so I can't set up sshd)

I already have Droidedit. Now I can build PETTIL on my phone while waiting in line at the bank. I haven't figured out how to get VICE or C64 Emu to work yet, though I've actually seen a PET screen under C64 Emu from here. But the keyboard scan is mapped out to a C64, not a PET and I don't know where to put or how to apply the .vkm files yet. If I were doing C=64 development, I would be all set.

I tried. I really need good C64 emulation for my work, though. All the emulators I've tried are geared for gaming and don't have decent keyboard and disk file support.