Digital BW, The Print

An inexpensive way would be to use foamboard cut to size and binder clips. 
Attach the top of the print to the
footboard with two clips.  The 'handles' can also be use for hanging (with 
push pins).

You can even get acid free foamboard, but it is removable in any case.  If 
this style fits the 'mood' of the venue
it works well.  Looking at sizes on the Dick Blick site it looks like you 
might need to get buy a larger size and trim
it to 22x30.  Even the more expensive stuff (example: Elmers Acid Free) is 
less than $7 per board at regular Blick
prices and there seems to be a 25% off sale for the amount you say you need 
(and free shipping).

Just my $0.02
Kerry