I'm not sure how much current the Ultimate FX sound/control board will put out, and whether or not this would limit the number of LEDs in the blade it can light up. Others may well be be able to advise on this.
A custom made string blade would be relatively expensive - more expensive than getting a regular Maul FX saber with a longer blade to replace the saber with something that it sounds like would better meet your needs. (SpaceWindu has really refined and pushed forward cutom LED string blades and sells them, the prices reflecting the time and skill that goes into these
String Blades Built for Crystal Focus LS)
I guess cost is a key factor as you're looking to use and mod the saber you have?
That being the case, the options available that
may be cheaper than replacing with a regular/signature Maul FX or getting a custom made string blade would seem to be either making a string blade yourself (lots of work, great to do, info available on the forums such as here:
How to make your Makototsai string blade w/ scrolling effect ) or looking into the possibility of using and modding as required a regular FX blade with the Ultimate FX hilt. This might be available as a fairly cheap 'leftover' where folks have converted MR/Hasbro FX sabers to an in-hilt LED setup. but this would depend on the Ultimate FX board being able to power one of these blades - dunno if it could or not.