Nick - the use of the word 'custom' in relation to Technics keyboards can be confusing to me and others because the Technics Company decided years ago in their wisdom to produce a very limited number of extra style files and create a specific space for them in the keyboard called the 'Custom' rhythm group - see rhythm group buttons one in from the right in the lower set.
I rather suspect you are using the word 'custom' in its general sense but you would need to confirm this before we can go any further, please...