I'm going to have to issue another Football Statto nerd alert before I post......
......but the playoffs were introduced in 1987, and they actually only had the concept of including teams that were in the relegation places for two seasons.
The playoffs those first two seasons included the teams that were 4th from bottom in the upper league, and the three teams that were just below the automatic spots in the lower league. All games (including the final) were played over two legs.
In fact, over the six playoffs that ran using this format (2 seasons x 3 leagues), the team from the higher league only won once (Charlton in 87), the other five times they were won by a team in the lower league. The reason it was abandoned is that it was only ever supposed to work like that for two years anyway - it was the time when the top division was being reduced from 22 teams to 20 and they didn't want to straight up relegate 4 teams from the top division each season.....they wanted to give the team 4th from bottom a fighting chance. By the 1988-89 season, the numbers in each league were as they are now, and they adopted the playoff system that we still have today.
(as an aside - this is why having an odd number of teams in League One this year is not unprecedented.....in 1987-88, there were 21 teams in Div 1 and 23 teams in Div 2 because they were halfway through this process).
But it's not a bad idea for how to finish the season if they get to start playing again in late May or early June. You'd probably just have to agree on Points-per-game as the way to account for the fact that not everyone has played the same number of games.