*long comment*
The main problem is that there is a g’zeira not to support/hold (ma’amad) the schach with something that is itself not kosher schach because chaza”l were afraid you will come to rely on that item for schach itself. They did, however, allow one to support the supports (ma’amad d’ma’amad) with such an item.
In general, string, cable-ties, wire, etc. cannot be used to support the schach and therefore if one isonly concerned with an abnormal wind so he may loosely tie it as it is not actually doing the job of supporting the schach during normal conditions.
If one is concerned that a regular wind will blow off the schach then this would not be permissible. There are 2 solutions to this problem:
a) Use the “woody” string that they use for holding together the schach mats – sometimes sold seperately. It is a meterial that is technically kosher for schach. This is cheap but not that strong.
b) Place wooden supports directly on top of the schach in the same place as the supports under the schach with the schach sandwiched in between. Now you can tie the upper and lower supports together with whatever you want as this is ma’amad d’ma’amad.
P.S. According to the Chazon Ish, one should preferably not rely on ma’amad d’ma’amad. But it is kosher nevertheless. (He would have made a brocho in a “non-Chazon Ish” sukka.)