We are looking to adapt the successful feedback mechanisms from the Staging Ground to work at the scale of the main site.

First of all, there is no "scale of the main site" anymore. The number of asked questions is extremely low and there are enough curators who more than capable of handling them all.

Now, to the main point.

You already have Staging Ground. But you never fully made it work, regardless of the feedback you received for it.

Main problem with SG is that not enough experts in the subject are able to see any particular question and give feedback on it.

First you made SG optional, so not all users see those questions at all. Considering that without SG those questions would end up being directly asked on the main site, there is absolutely no reason that SG is optional.

Next, SG question which has been reviewed and not published will vanish from the timeline. this makes those questions even less visible and less likely to be published if the question is sufficiently edited by the author.

For some questions early feedback can be given by many curators, but when question gets edited, only those who did initial review will be notified. But if the reviewer is not SME, it may happen that they are not in a good position to review the question again. Publishing question prematurely can have a negative impact on its reception. On the other hand, not publishing it means it will be stuck in SG without an answer.

If you would show those questions in the timeline, just like any other question, regardless of its review status, then actual experts would be able to see them, give feedback or publish and answer them.

All this would be extremely easy to implement.

Also you could give all users a choice to publish questions through SG, so those that really want feedback could get it.