What you suggest will probably be OK. CAT5 cable contains four twisted pair, each with a different twist pitch. This causes almost all coupling from one pair to another to be common mode. It's really hard to inject a signal into a CAT5 cable that couples in differential mode to a different twisted pair.
RS-485 is a differential signal going from up to +5 V to -5 V when transmitted. Receivers have a couple volts or so hysteresis typically. It would take a lot to exceed the common mode range, or introduce enough differential mode noise to prevent correct reception.
One thing to keep in mind is that while RS-485 is differential, these two lines still need a ground reference. At a minimum, this requires 3 wires in the cable. You probably want to dedicate a whole pair to ground in reality.
800 kHz is really not that high. Unless the voltage swings are very large, it's simply not going to cause problems for RS-485. Since 800 kHz is your highest frequency of interest, it might be a good idea to add a filter in front of the cable to attenuate any higher frequency components, which you don't want anyway.
If you can send the 800 kHz differentially using its own twisted pair, even better. Even just a balun right before injection into the cable will help, although I really think you'll be fine even if this is sent as signal and ground on its own twisted pair.
I really don't expect any problems with what you are propsing.
