Panniers weren't yet part of the French fashion scene, so you're spared that nightmare.  I think you could do a "rehearsal" "Tartuffe" - along the lines of the Gielgud/Burton "Hamlet" back in the sixties.  Have the characters dress in modern clothing that emulates the lines of period clothing, with colors that fit the characters' personalities.  Like a cast would do while rehearsing a play - the ladies would wear long skirts to get used to moving in them.  The men would wear jackets and capes and rapiers and walking sticks, etc. - to get used to using them.  Have your set an unfinished shell, and do a lot with lighting and music.

Another approach - if your heart is set on full costume, but the expense of multiple costumes for all the characters would be prohibitive.  Begin your play in modern dress (jeans and sweatshirts and the like, using make-do items for props and costume pieces), and as the play progresses, add bits of costuming, props and sets - sort of like you would see happening during your multiple weeks of rehearsing - so that by the last act everyone is fully costumed and the set is completely dressed.  You could even go so far as to have the actors carrying their scripts in the first scene, gradually dispensing with them as they "learn" their lines.  This would give the audience  a view of what goes into putting on a play, and the additional fun of watching the play "come together".

Mmmm Some really nice ideas here. A kind of "Rehearsal Tartuffe" is pretty much what I originally envisioned. I guess the question is how to create the look or outline of a mid 17th c. costume with modern clothing.

The idea of costume notes is interesting. May be something to try.

And as for the panniers, that was really my big concern. I see it in so many Tartuffe costume designs, though.

You might check Drama Exchange. They are a pretty new resource for costumes, sets, props, etc for theatrical productions. You can both buy items or sell things you don't want anymore. www.dramaexchange.com