TTL is TTL. The Oncore is TTL. It all really depends on what is meant by inverted TTL wrt to RS232..

If by inverted TTL you mean that 'Serial 0' <=> 5V, and 'Serial 1' <=> 0V, ie it's inverted 'serial' logic at TTL levels, then you'll probably need a couple of inverters. (Hint - Often a Quad Nand gate is easier/cheaper to obtain than a dual/quad Not gate...)

If by inverted TTL you mean that 'Serial 0' <=> 0V, and 'Serial 1' <=> 5V, ie it's inverted 'RS232' logic at TTL levels, then it should be plug'n'play.

One thing to beware of though, is that since the tuner port is TTL level, it's circuitry protection will far less hardy than the real serial port (serial transceiver chips are almost bombproof). 'Real' TTL can be damaged by >6.5V. I'm not certain, but ISTR that all of the empegs serial ports are onboard the Arm CPU, ie no separate serial UART. If this is the case, then blowing the tuner's serial port might be synonymous with blowing up your CPU, ie expensive.

You might want to get the Empeg guys to weigh in on this.