Could an old, unconfirmed transaction fail to be processed because it is no longer deemed to have enough fees I just read Why do small transactions without fees ever get processed? and it made me wonder about the risk that a legitimate, lock_timed transaction might not be processed once the lock_time is past.

I'm aware (from the above question) that in the reference implementation, the increasing age of a transaction (in the mempool once it's been submitted?) should increase its priority regardless of the its fees but not every miner is willing to process low(/no) fee transactions even now never mind what they may (not) do in future.

So if I have a signed transaction with a (very!) distant lock_time and it has fees that were considered reasonable at the time that the transaction was created and then when I submit it after lock_time what is the risk that no miners will process it because its fees are subsequently considered to be too low to bother with it?
If that does happen, all you need to do is spend your unconfirmed transaction's output with a sufficient fee. A "sufficiently greedy miner" will process both transactions just to get the fee of the latter.