I have no personal experience with a 102mm aperture but my 400mm can do useful work on a 1% depth at 13th-14th magnitude. Simple scaling says that you have 1/16 the collecting area, which corresponds to three magnitudes in brightness.
Accordingly, I expect that you may have a chance with HAT-P-20 but it’s not certain. Certainly well worth giving it a try.
Regardless, it’s easy enough to test the capabilities of your equipment. Choose a star of a particular magnitude and see how long an exposure is require to reach a SNR of at least 500 and preferably 1000. If it is a couple of minutes or less, and preferably less, you can measure a transit of depth 2% or better. Repeat with fainter stars until you determine your limiting magnitude.
Good luck!
Paul