I totally understand your point but note that a lapdock will typically get you a touchscreen whereas a laptop may not (and in the case of the Librem 14 certainly does not). So there’s a difference in experience, which may or may not be important in your use case.
Another approach would be simply to do the task on the laptop. (So that’s another difference between lapdock and laptop i.e. the laptop is an independent computer in its own right.) Whether this is viable in your situation depends on what the task to do is.
Finally, the phone can export either or both of its disks using a variety of protocols if you want to mess around with files that are on one of the phone’s disks from the laptop.
None of that answers the question you asked though.
Good suggestion. Sounds “advanced”. It seems to be possible to “program” the USB on the phone end to advertise all sorts of interesting things. Is it possible instead to do something similar on the laptop end? Would be good to know.
Even if you can do this, there may be limitations to how well video can be sent. So it probably won’t work at all via DisplayPort altmode - and it could depend on which USB port is used on the laptop.
Assuming that this can’t be done currently, I think this would be a cool feature for a future version of the Librem 14 to support.