To be honest, I didn't expected it to be that complicated to fix, with a lot of trials and source reading (on my shinynew Cybook) to figure out why the code behaved differently than I expected. But at last, AnimEditor is now able to set "sticky limbs" on the first frame of an animation as well, which in turn enables me to craft a walking animation for Bilou in the editor.
I took the time to document the thing a bit more, too. The trick is that I have two representation for the animation: the tlist that is a sequence of TIFrame at specified time positions. This is just perfect for editing, as each frame fully encode the state without any reference to past/future state. The game engine needs just the opposite: a list of commands that instructs it to move that limb, change the sprite used by this one, then move the whole sprite by x,y pixels and wait for 3 vertical refreshes. All these commands are encode()d into flat 32-bit integers that are easy to read and write to files. When exporting them, frames are compared and updates are generated when required. When importing, the last generated frame is copied after every delay and altered to reflect the new state. So far so good.
It gets a little more complicated with the "sticky limbs" thing. The editor needs to know which limb sticks at the same place on-screen. You do that by selecting one of the limbs on the leftmost "limbtable" and then pressing the "time/move" button just between the frame editor and the animation preview (highlighted on screenshot), which initially just reads 'time'. If you decide to move dumblador's sticky foot further to the right, dumblador will move faster, and if you move the foot closer to dumblador's body, it will slow down its walk.
At the export, the offset of the sticky limb compared to the previous position of that limb is used to generate a "I_MOVETO" command. Devil was in the details, as always: when should the information be cleared when reading back, how to handle the commands when a TIControl item is selected, and whether the additional I_MOVETO has to be stored at the end of the serialised animation or before all the other update commands.
Well, I got it more or less sorted out now. All I'll need is improve the walk animation itself and patch bilou.cmd so that it makes good use of it.