Mai does have a few options for HD combos without the use of super meter stocks (0 stocks). Mai is also capable of following up after the HD meter disappears.
Mai problem[corner] s.B, s.D [HD] s.D, hcf D [HDC] air.qcb C, hcf D [2hits] [HDC] qcb C, hcf D [2 hits] [HDC] qcb C, hcf D [2 hits] [HDC] qcb C, qcb C, air throw [597]
This was sample of what "can" do after she the HD combo. An air throw results in a hard knock down and she benefits from hard knockdowns.
If you wish to save meter, than this should be a good HD combo to use. But, there are plenty of methods of following up "after" a HD combo. Instead of an air throw you can do a reset with s.C and the continue your assault with some sort of high/low mix up.
If you have some meter to burn (if you want to finish a match quickly) you can one more EX move or DM to keep your cycle going.
If you have two meters at your disposal, you can use the EX Chou Hissatsu Shinobi Bachi (Her EX DM) instead of the Air Throw. This will result 680 damage.
If you have three meters at your disposal, you can use the Neo Max which will result in 784 damage. You need to do this on the very last Ryuenbuu loop and input the Neo Max notation when her arm smacks the opponent.