The RCHourglass transponder can be easily converted to send a 3.68MHz signal (or 3.58MHz...these are 'standard' values). The crystal must be changed from 5MHz. The antenna must be retuned for the new frequency (different number of turns and/or capacitors).
The problem is finding the packet data that must be sent, and changing the transponder firmware data. Also (small change) fine tune the delay time between packets since it's related to the system clock.
How to find the packet(s) sent? Use an oscilloscope and/or run a modified version or the RCHourglass decoder that runs the front end at 3.68MHz. The decoder probably needs a 'relaxed' preamble detector and 'Monitor mode' must be used. It's also possible that the transmitted telegram is shorter than RC3 transponders, so the trailing part of the packet seen in monitor mode is probably garbage.