Progress : I finally managed to wrap my head around the interactions involved in a summon.
After a few unsuccessful tries, I copied over the summoning from masterwork and edited it to use truetransform.
Here I summoned dogs as a test. I still have to fix the messages (Milith McSuccubus transformed into a nothing) and I used a duck midway.
Sadly you need to lock the door to the circle too because the worker often leave before it is completed, causing the summoning to fail. I need to properly immobilize the worker until it is done. Perhaps the timers can use a change too so you can chain them without having several worker break each other jobs.
Last thing to check before I add proper summons, the creatures summoned are 'friendly'. You can move them to pets by typing 'tweak makeown' in dfhack and get access to training, I'll try to have autosyndrome do that for you.
Edit : I fixed the messages and added a dummy creature for the transformation. If I can fix the 'friendly' part, we will have the summoning AND invader conversion ready. A strange thing is that you often end with 2 summoned creatures, as the worker tent to rip a part of the dummy creatures and raise both. I like it that way.
Also using tweak makeown on those creatures cause them to be untamed but non hostile, you'll need animal caretakers. (I tested it and it works)