We'll see if others have current experiences with this, I know there are many members with these types of issues with church attendance. One thing I'd note is that in the past few years, sacrament (and other church) meeting broadcasts have become readily available from home, something that was not previously possible in most places before the pandemic. So for many situations, the member may be able to participate virtually and have the Bishop's permission for the sacrament ordinance to be administered at their home, which avoids the time and resources required to set up and maintain allergy-proofing at the meetinghouse. And it may be that the FM Group has made policy changes due to the widespread availability of remote worship.
I would expect that if building modifications were desired, the Bishop would bring this up with the Stake President since he's over the buildings in the Stake (or the Bishop could go through the Stake PFR who would then talk with the Stake President). And the Stake President would then contact the FM Group to initiate discussions about making any proposed changes. Hopefully there are Forum members who can share current-day experiences regarding the available options for what you're asking about, and current FM Group policies.