This is quite a novel idea, but you would probably still require the mcu to drive the DTMF transmitter.

One solution is to use a LM567 (may be obsolete) to decode the DTMF and drive a retriggerable monostable (LM555 may do the job). The output of the LM555 can drive a transistor followed by the relay or the relay directly.

The DTMF transmitter now pulses each relay that you want ON for say 10ms, the monostable will then hold the relay for say 1 second. So when the monostable is retriggered the relay stays ON for another second.

A good (or bad) feature is that if the DTMF stops all the relays go OFF.

Andre