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No restriction on pawn uncaptures (except that it can't be done on first
or last rank) could probably lead to a too drawish game. A player just
needs to delay pawn uncaptures until all other pieces are uncaptured and
than try to uncapture all opponent's pawns on the same file. 

So, I would suggest the following rules:
* Pawn uncapture is not allowed on the file where there is already pawn of
the same color. For example, black can't uncapture white pawn on g-file if
the file already contains another white pawn.
* Uncapture is also not allowed if it produces a pawn, which is behind
player's pawn. For example, black can't uncapture white pawn on g7 if
there is already black pawn on g2. 
* A player can't uncapture more pieces then present in usual chess setup
(light and dark-colored bishops are considered as different pieces).
However if a player unpromotes one of his pieces, that piece becomes
available for uncapture again. 

The game finished when:
* A player reaches starting postion with his pieces (this player wins).  
* A player can't make a legal unmove (this player loses).

There is already a ZRF file for this game without any restriction on
uncaptures: 'Unplay Chess' by  Karl Scherer (can be downloaded from
Zillions of Games pages:
 
http://www.zillions-of-games.com/cgi-bin/zilligames/submissions.cgi/26414?do=show;id=966

I slightly modified it to place white king on a1 and black on h8 in
initial position. Here is my first game against Zillions (I played
black):

-1. kg8 (*Rh8) // black uncaptures white rook: kg8xh8
-1. . . . Pg7 (*rh8) ?? // white unpromotes the rook and uncaptures the
black rook, undoing the move P g7xh8 R. This move loses the game.
-2. rh7 (*Qh8)! // undoing rh7xh8. Black wins as white doesn't have any
unmoves which wouldn't leave black king in check by queen on h8.