The show is on from 22nd-25th July and a quick look at RTT I couldnt see anything yet for the extra train. However it may be a last minute thing on RTT.
It wouldnt be too much of problem running via Landore and Llanelli, what might be the bigger issue is where TFW are going to find a spare pair of 150s or a 150/153 if its to be 3 car. Also the regular HOWL service get doubled up to at least 2 car (those that are booked single 153s) during the Royal Welsh show and with 153s being used on more Valley Lines now e.g Caerphilly to Penarth and Coryton to Penarth, they are pretty stretched as it is.
Lets hope if it does run it will be more successful than the last couple of years!