I'm very interested in what you find out.

First, why are you replacing? Is the Cat crushed? Or punctured? Or rusted? The shell can be repaired by a welder if you remove it from the car. This would only be worthwhile if the Cat is actually still functioning inside though....

I would love to know what aftermarket Cat would be an acceptable replacement to the fuel computer.

You won't do damage to your engine, though the computer could give you a bit of a rich mix if it thinks the Cat isn't working properly.

But I am wary of whether the computer will accept a Cat with different characteristics. I know some of the performance guys are using aftermarket Cats but most of them are also using tunes or even completely new fuel systems, so their experience might not be of much help in the quest to find a suitable replacement for the stock fuel computer.

One option would be to get a universal Cat, have it welded in, and then space out your rear O2 with thread chasers. There is plenty of info on this on the web and it might turn out to work?...
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1998 V70 AWD->FWD->AWD Turbo 220k+