Consider me another requester. Since Apple already supports keyboards where they can (for things like Copy and Paste and Undo), I feel Pleco should, too. Also, the fact that there weren't many requests so far doesn't necessarily mean that users wouldn't want a new feature. Sometimes they will love something they didn't know they wanted, because they will only know it when they can try it.
On the implementation: I think Pleco could use the number keys 1, 2, 3, and so on to support pressing the buttons from left to right (like Anki does on PC/Mac) or top to bottom, perhaps the Tab key for jumping between different blanks in Fill-in-the-blanks tests (it might already support that), and the Enter key to confirm the blanks. For Tone Practice, just pressing keys 1-5 (or 6 for Cantonese) should be enough to go the next blank/question and perhaps the backspace key to re-activate the previous syllable.
But of course, mine is just one of many opinions. I wouldn't be disappointed if you were to make a different decision—it's always the programmer who decides.