The rules (and implications) around moats being bridged are relatively complex. (Relative to the general framework of chess.)
Why not simply make the moats permanent? Two player chess has four corners, this variant would have six... seems about right. Albeit that I haven't played this variant at all what breaks in the later game if the moats are there?
I realise that it would limit the circle for rooks and queens in the outer rank, but in playtesting does that come out as a bad thing?
Yea--would be easier. But many games end without a moat ever being bridged. When you do have a game that goes on & on, to where there are so few pieces as an outer rank becomes completely vacant, it helps get things moving, and adds some excitement. The original discussion was whether to at least, leave them bridged permanently, once they ever become bridged. (instead of having them open and close). The reason for that was so a team wouldn't have to hold pieces back, just to keep their moats from being permanently bridged and committing suicide.
I run a chess program for children ages nine through 11. Most know the fundamentals of chess but little more. The moat and creek rules are beyond their comprehension so I tell them both are permanent. It does not detract from their game.
Are bridges one-directional? i.e. white moves all of their pieces out of Rank 1, now both black and gray can move pieces over the moat into white's section... but can white also move their pieces over the moat into black or gray's section?