TL;DR: Should we only use material created during the 48 hour window, or how much previous work can be used?

My team has begun discussing what we want to do and who it going to do what.  However, none of us has done a tabletop jam before.  We were wondering what kind of pre-work is expected or considered inappropriate?  Especially given the scope of a full, (hopefully) publication-worthy game, 48 hours is a tight timeframe.

We've discussed a few ideas for core game mechanics, looked at a few SRDs, and started to get our heads around a system that we could all play.  Is this expected / reasonable?  How much is too much? 

Our artist is planning on generating custom works for the game once the theme is announced.  But she'll likely pull works she previously made if they can thematically fit.  Should we limit art to what is generated in 48 hours?  Or is reusing previous materials expected?

I expect backstories, setting info, and other prose to be written once the jam begins.  But what if a team member already has short stories or descriptive sketches that fit the theme.  Is it ok to incorporate those or not?

These are all good questions!

We've discussed a few ideas for core game mechanics, looked at a few SRDs, and started to get our heads around a system that we could all play.  Is this expected / reasonable?  How much is too much? 

This is totally fine! You're more than welcome to talk systems and even take notes about the kind of game you'd like to make. Writing the actual text that your game would use to describe these mechanics is where I would draw the line. That part should be done during the jam itself.

Our artist is planning on generating custom works for the game once the theme is announced.  But she'll likely pull works she previously made if they can thematically fit.  Should we limit art to what is generated in 48 hours?  Or is reusing previous materials expected?

I've included links to stock art in the Useful Resources thread, so incorporating works made prior to the jam is OK. Making art specifically for the jam in advance of it is not, however. All art made specifically for your entry must be done during the jam.

I expect backstories, setting info, and other prose to be written once the jam begins.  But what if a team member already has short stories or descriptive sketches that fit the theme.  Is it ok to incorporate those or not?

This one is more of a no. If you happen to have past written works that fit the theme, you are more than welcome to use them as a jumping off point or as inspiration. However, all of the text used by your entry should be written during the jam itself (and not copied from a previous work).

TL;DR: Some prep is OK. For the most part, all work should be done during the 48 hour window of the jam.

Hope that helps!