Can I use an IR-over-COAX enabled modulator straight to the RLRF210 target?

Unfortunately, no. You must use an RLRF380 splitter/combiner, even for a single TV output.

The reason for this is voltage requirements, and not wanting to damage TV tuners or set-top-boxes (STBs).

An IR-over-COAX system works by passing the IR signal as a series of voltage pulses through the coaxial cabling, embedded on a constant voltage rail. 

With the Zycast/Resi-Linx modulators (like the Q4KR1, or HD4797, or RLDM1102, etc.), they have the ability to decode those pulses and reproduce the IR through an emitter, controlling the source equipment (like Foxtel, Blu-ray player, etc.). These modulators run from 12V DC power supplies and push 12V DC voltage rails through the RF modulated outputs to power up the rest of the system.

However, the RLRF210 targets work on a 5V DC voltage rail, not 12V. So they need something between the modulator and themselves to reduce that voltage to the level that they want. In this case, it's the RLRF380 splitter/combiner. The RLRF380 takes the 12V DC rail supplied via the MOD RF IN connection (don't use the local power supply if you want the IR to work), powers itself (for a small amount of signal boost), and converts that to 5V for the eight RF outputs.

The much older Resi-Linx IR over COAX systems didn't work this way, but there was a potential problem.

If the installer or customer did NOT use DC blockers (like the RLACC130) on any TV point connected to the RLRF380 that did NOT have an RLRF210 target, the 12V DC voltage passed straight into the TV tuner or STB input. Feeding power into a socket that isn't designed for it is a great way to break things. It was never a large amount of power, but some equipment wasn't happy about it anyway.

So to avoid potential damage to the customer's equipment when not installed correctly, the targets were altered to 5V DC and different overall power levels, with the RLRF380 doing the important job of keeping that power in check.

The downside being, of course, that you always need to supply and install an RLRF380, even for single-output, IR-over-COAX systems.