Maniphest should allow you to reasonably prefill the "create" form with GET parameters. Some fields already work but others don't. In particular:
- All fields should support this.
- Fields with tokenizer values should accept readable values rather than PHIDs ("assigned=epriestley", not "assigned=PHID-XXXX-YYYY").
- Custom fields should have a hook for reading values from GET.
- There should be documentation about this, and possibly some sort of automatic UI integration (e.g., a view where you see what parameter to use for each field).
The "?template=x" parameter can also be used to do most of this, and might be a better starting point.