A 1984 Carrera had a side mounted oil cooler to give one example.

 

there have been pics of trannies with the oil cooler but again it was complicated and expensive. I really tried to get it done but no one had any great way to do it, and most porsche guys I asked didnt think it was worth the trouble or money.
I do change the trans oil fluid pretty frequently though.

 

The MSRP for the GT2 transmission (G96.88) oil cooler parts (brackets, pump, cooler, etc.) is ~$1500USD (so you can probably find it for half that) but the parts are listed as,
"THIS PART MUST BE RE-ORDERED IF IT IS UNAVAILABLE WHEN THE ORDER IS FIRST PLACED. ORDER BACKLOGS ARE NOT PROCESSED."
So did you check with Porsche?

 

I've got a quote from Techart (their swiss dealer) for a gearbox with reinforced 1-2-3 gears and oilpump. Price here is around CHF 18.8k (USD 15), including installation.