Well, this was a spectacular fail...
My 'bot started extruding, i've let it extrude into air for a while, to see how it's behaving.
Then zeroed in the Z axis (i've got the end switch at the MAX end of Z, so the procedure consists of homing Z, then lowering it until it's a paper-width away from the glass plate and issuing a G92 Z0).
Then retracted the filament a little and let 'er print the 0.5mm thin-wall calibration object.
So it starts extruding the skirt and it dots around a little in corners but then starts extruding nicely, moves towards the center, does the first layer...
All hell breaks loose.
It manages to extrude the first layer, starts the second, makes a tangled stringy mess, at which point the extruder gets stuck, so i kill the print.
I extrude a little manually, only to find out the bolt unscrewed itself out of the locknut. I try to tighten it (retracting then tightening the bolt with a wrench does the trick), but it unscrews itself again the moment i try to extrude. I realize at this point the filament is well and truly stuck in the nozzle, so i lift the Z up so i can have better access to the extruder and one of the Z axis rods pops out of the coupler (i was experimenting with "soft" coupling, by sleeving the Z rod in cable sleeve, then inserting that in the coupler).
As if that's not enough, i try to retract the filament out of the extruder completely, except it gets stuck inside. I wait for the hotend to cool down a bit, disconnect everything and remove the extruder from the bot, but getting that filament out is a no go. I finally separate the hotend and manage to get the filament out, clean the head with acetone and poke the nozzle with a needle and it finally seems clear. Except i look through it and find out there's still a chunk of filament in the SS barel that connects the nozzle/block to the cold end.