|Cult of Vorg|
Saying the grappler must maintain 1/rnd after the first sets a minimum, but doesn't prevent extra maintain actions at any point. So it doesn't say you can't, and the action economy works..
However, 1-round hogtying doesn't sound fun for anyone at a table except maybe that rodeo grappler. So, I'm inclined to be harsher, and say no to that until there's a rule that specifically allows a grappling maintain check more than once a round, instead of just reducing action econ until it would be possible if allowed.
Also, I don't think pinned, with or without being tied up, makes someone helpless. The example of tied up as a reason for helpless I believe would have to be a much more severe case, full restraints not just a little manacle-ing.