I'd like to restrict whether a particular project appears on my report based on whether a specific user is a member of that project. I can easily get a list of project members to appear in the report, but cannot see any way to apply a filter the way I want (i.e., there seems to be no way to either query for a specific member _or_ display all the members as text in one field (concatenated) so I can user a standard filter. Anyone have an idea?

Not entirely sure what you are seeking so...

You can use a (or multi-select) prompt to produce a specific (or list of) Enterprise Users & EnterpriseMemberIDs. 

• REM: change the filter [Presentation].[Users].[Full Name] = ?Prompt? to [Presentation].[Users].[Full Name] in ?Prompt? with a multi-select

the following two fields can join Project and Enterprise User information:

• Project Information-->Project Member-->IDs-->EnterpriseMemberID
• EnterpriseMember-->IDs-->EnterpriseMemberID

the following filter may be applied to a project query:

• [Presentation].[Project Member].[EnterpriseMemberID] in  ( [PromptQuery].[EnterpriseMemberID] ) 

I hope the above is helpful in getting to the report information you want.