It is easy to file a Eurocontrol flight plan at some low level (e.g. 2400ft).
The computer doesn't check a string of DCTs, except for the MAX-DCT value which in the UK is 100nm below FL100, I think.
The problem is that it is worthless. London Control will wash its hands of it within 100ms of you pressing the Send button
It will be ignored by all enroute units, and (apart from telling people where to look for wreckage) is worth no more than just flying non-radio all the way, "VFR", having made a phone call to the destination to make sure it is open, etc. (although if you file a flight plan to a destination which is closed, due to wx or a crash or needing PPR/PNR, etc, they will invariably
not return a message saying you cannot go there
).
The grey area is filing IFR flight plans at some "not quite obviously airways" level e.g. FL060. This can break down (in terms of acceptance by enroute IFR ATC) for various reasons too, starting with the departure airport giving you a squawk which belongs to their allocated squawk range and not one they got from Control. I thus never file IFR below FL100 and preferably FL140 because that's where the routes get better.
I've got some www notes on this stuff.