When it comes to buses, I don't know too much about transmissions and such, but most manual transmissions in smaller vehicles do not have full pressure lubrication. This means that to utilize an oiler cooler one would have to install a pump to circulate the gear oil.
Have you checked to make sure the trans is not low on fluid??
IF you found a good electric pump for moving the gear oil, it would be relatively easy to connect fittings to the fill and drain holes in the trans and run lines to a cooler and back. A heater core from any passenger vehicle would make a lovely trans cooler i would think. Check with west marine (http://www.westmarine.com
??) and see if they sell a pump that will move oil. The trans cooler should be mounted where it will get plenty of wind when the vehicle is moving. If you utilize copper tubing instead of rubber, that would also help with the cooling. .