I believe you could try doing this with
KeyBoard Hooks[
^]. Install a keyboard hook for that process, watch for the key hits witch correspond to 'u' and 't' or whatever you choose the keys to be and try changing the key codes to whatever keys would normally move to the next page and the previous page (can't tell that by hearth but they probably exist). So basically if the user would press e.g. 't', your hook would convert it to -let's say- 'space', so when the viewer pp processes the message it "sees" that 'space' was hit and moves to the next page.
What might be tricky here is that this will also modify the key when the user isn't looking at a slideshow but e.g. has the 'open file' dialog up and is trying to type in a file name (if the pp viewer has such a functionality, can't tell that by hearth either).
Good luck.