Volvo actually had a recall on the plastic floor trays for the 700's (identical to the 900's) in the early 1990's for exactly that problem. The mat could slide forward and trap the accelerator pedal. The remedy was a two pronged clip that attached under the humped plastic cover at the front of the outer seat rail. The revised floor mat then had two slots in it to fit into the clip tangs. It was a good solution except any vicious tugging on the clip would break the plastic seat rail covers (I've said good-bye to a couple of those over the years).

When I wanted to get some mats for my 940's I went to a very knowledgeable local yard. They had never heard of the clips let alone a recall. I made them go looking and sure enough they found the clips and even one pre-punched mat. For the other mat I simply punched my own holes. I even did the passenger side, but that really isn't necessary. I'm sure there's a Volvo part number for these clips and they might even still be available, but cruising the yards would be your best bet. The only other sure fix is to screw the mats to the floorboards or firewall -not exactly a great solutuon.

Like Steve says, the best thing to do is make sure that 700/900 drivers are aware that mats can slip and get caught up in the pedals.
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Dave -940's, prev 740/240/140/120 You'd think I'd have learned by now