Re. other freqs - assorted spots all over the 27-49MHz spectrum find "unrestricted" use for RC toys & model aircraft,cordless phones etc, & these lower freqs naturally have better "punch" thru' terrain & vegetation. Circuitry is pretty undemanding,although antenna will not be such high gain. A bonus with 27MHz is that old CB sets & maybe even kids 3-4 transistor super-regen. toy walkie talkies (selling here in NZ for ~US$5 a pair) could be hacked for parts- especially the crystals. Additionally most decent SW comms. radios cover that part of the spectrum, allowing easy listening to your outgoing signals. Yah!
I'm short of time to explore regs., which of course could be country dependent, but suggest some bright spark check further. AFAIK the 27 MHz band is legal in most, if not all, countries for use with all types of RCs, but may be subject to interference from the adjacent classic AM/SSB CB band. Cheap toys seem to usually use Channel 4 if there is no channel stated, & may use resistor style colour coding system.
* 26.995 MHz -- Channel 1 -- Brown
* 27.045 MHz -- Channel 2 -- Red
* 27.095 MHz -- Channel 3 -- Orange
* 27.145 MHz -- Channel 4 -- Yellow
* 27.195 MHz -- Channel 5 -- Green
* 27.255 MHz -- Channel 6 -- Blue
Mmm- just recalled this snippet- regs. for some freqs. here could be worth checking? The allocation 433.05-434.79 (centre 433.92MHz) has been an ISM (Industrial, Scientific, Medical) band for many years, 433.92 being harmonically related to 6.78, 13.56, 27.12, 40.68 MHz etc. Dividing by 64 we get the 6.765-6.795 band, and by 32 the 13.533-13.587 band.
Yet another approach may be via low power FM band transmitters- the MP3 "iTrip" units now abounding naturally tempt. They have only a few metres range as is of course, but are very stable & maybe be great "engines" when amplified. Stan