Yeah I would think a 40-man spot is still worth less than Walker's salary. It's a sunk cost but that being the case you just view him like a guy on a minor league deal. OTOH there's no chance he'll be claimed. I wonder if he would jump to declare free agency and start over or if he does want to try and redeem himself here. If the former he would then lose access to training and facilities until he signed a minor league deal somewhere else.
I don't think you get a free pass on Walker's roster spot, he'd still have to be DFAed. Last year they put Covey on the IL to make room for Plassmyer and then optioned Plassmyer to bring up Nelson. But those were on the last two days of the season.
What you do get is a free pass on the minor league recall window, if you wanted to option someone and then bring him back on the playoff roster.
My guess is Walker still pitches on that day. If it feels important to win it they should consider an opener followed by Allard (or Banks if it's a lefty-heavy line-up) and then see where it goes.