SMRT's RFC routine indeed cannot handle alternative-specific designs. We figured out how to make it work in our newest Online Market Simulator.
That said, if you have individual-level utilities (via HB, for example), the Share of Preference (logit) simulation method should work fine, and I'd recommend you use it if you need to stick with SMRT for this project.
The tuning of the scale factor indeed works very similarly as with RFC. Adjust the Exponent to make the shares steeper, relatively, or flatter.