You ask a very interesting question, and one that the EFL's own rules, as published online (remember there will be a legally enforceable version elsewhere) are vague about. Here's the bit, in section 6 on 'Players'
Rule 53 Standard Loans
53.1.4 A Player whose Standard Loan expires (or terminates) at the end of the Season of the Transferee Club but prior to the completion of the League Matches of his Transferor Club (where it is a Member Club) will not be eligible to play in any remaining League Matches for the Transferor Club in that Season;
The words in bold are the key ones:
'Season' is clearly defined: ‘Season’ means the period of the year commencing on the date of the first League Match and, for each Club, ending immediately after the completion of the Club’s final fixture of the League Competition or, if the Club is participating in the Play-Offs, the final Play-Off match for that Club. So, and lets use my favourite hobby-horse Craig Bryson as an example, his SEASON with Cardiff has finished. All league competition fixtures complete, no play-off matches pending.
'League Match' however, isn't well defined. A ‘League Match’ means an Association Football Match in the League Competition. A ‘League Competition’ means the annual League competition for Clubs organised by The League. that doesn't make it clear at all whether the play-offs are included as a 'League Match'.
Failing the inclusion of proper definitions, even if the EFL MEANT to include the playoffs as a set of matches that a loan player couldn't return for, and IF a club (lets say Derby obviously) realy wanted a player back, I think they could argue that the LEAGUE has finished, and could cite the EFL's own records, which show that in seasons past the LEAGUE represents the 46 home and away games, NOT the outcome of the playoffs.
BUT:
Are we bothered? Bryson might have been useful had out midfield not started to 'tick' a bit, especially with the improvement in Johnson and Lawrence, Martin would be a 'last throw of the dice' player and how motivated would he be as fourth choice forward? Blackman, even if we were desperate, hasn't finished his season yet. And I nearly forgot Butterfield, which says it all