Not sure how much these facts'll help, but I've been testing some stuff with Ditto and can confirm:
- He does not copy hold item effects - YMMV on the impact this has.
- If he transforms into someone with multiple forms, he directly takes the form they're in.
(These two were tested with Ghost-form Arceus. Ditto transformed into the Ghost form, and it was immediately followed with "Ditto transformed!" and the sprite turning into Normal-type Arceus, since he didn't have the plate for it.)
- Quick Powder's effect does not last through transformation. (Arguable)
(Tested this with my Alakazam against a Glameow. 5 Shadow Balls and a Trick each, Ditto went first for 4 of them. Could probably do with more testing on this. (Obviously stopped testing after the Trick, since that meant Ditto no longer had Quick Powder.))