Well it use a module in order to have direct memory access through the kernel. A binary is then used to fetch information and flash updates.
It is "possible" but need a lot of work.
I try to find a workaround...
EDIT :
As a workaround, let DSM download the files, go to /volume1/@smallupd@te_deb, un pack the flashupdate deb, replace updater with a shell script 'exit 0', repack, replace and push the button. I can easy be scriptable.
The flashupdate is here to update synoboot, but we don't care.
Note : it may break your DSM, I suggest you to try on a virtual machine before.
I update from 3776 to 3776-3 and everything is working fine.