Though if the 10-day recall rule did not apply at the end of the season they could use guys like Kerkering and Sanchez on Saturday and option them out for Nelson and somebody else on Sunday. The rules online are kind of cloudy on this because postseason eligibility has nothing to do with optional assignment. Player just needs to be on the 40 or an injury waiver like Kerkering who also needs to be in the organization.
I do wonder if the Phillies petition the league to allow this. FWIW players do get major league pay and service time when they are optioned out in September anyway, so this would cost a little extra money.
I did find this exception language too on the MLB site:
But in September, another exception emerges. If a player is optioned to a Minor League club whose season -- including any playoffs -- ends prior to the conclusion of that 10-day period, the player can be recalled at that point, with approval from the Commissioner's Office.