When I first got my R3T I too really missed not having a tach. I purchased a Scangauge and used one of the settings for RPMs. The Scangauge refreshes pretty slowly so you can use the gauge for steady cruising, but for acceleration it does not keep up.
After a bit more than a year, I rarely use the RPM feature and find that I don't miss the tach. Unless you have money to burn I'd suggest you wait a bit and see if you still feel the need.