I hope you understand I wasn't addressing you specifically when I made my statements. It was more a blanket statement focusing on how I handled things.
With that said, you are the only one that can draw that line. You know your guest base, the capabilities of the operation, your staff and ultimately yourself.
The only answer I can provide would be one I could give myself or my staff. You just don't let that happen. Or if it does because the request is that off the wall, the person(s) that is(are) asking the special request(s) need to be just as accommodating as you are. It's a give and take scenario. If you don't handle things yourself because of the needs of the business, you have to communicate this through your staff to the guest. This would probably be better handled by the Dining room Manager or owner if you are too busy but they (the guest that is) have to understand that this may take a little extra time and if they could please be patient you will be as accommodating as possible. It's better than an out-right refusal and both parties end up losing something.
It's never going to be a total win for all but it's a starting point. The next time they dine at the restaurant and you have the time to visit the table yourself, do just that. Talk to them. Make this guest a priority to know. Just not too much of a priority. If this is a specific guest that is consistently making this request, ask that the guest to "phone ahead". It does put the guest on the spot and may curtail the request but you are strengthening your relationship with the very people that you need to let the business survive. From the sound of things in talking with you, the restaurant and area is just small enough that you could greatly benefit from this.
I'm sure we all know this but it's all about Guest focus and service. And meeting the guests needs is how we focus and serve the guest. I'm a fairly proud person and I certainly have my moments when it comes to ego. But the role we have taken, guest service, is a subservient role at best. It's anticipating the needs of the very people you need to survive and succeed. It is humbling and very frustrating but I also know it can be very rewarding. (Gawd I miss the rewards!)
So, once again, it all boils down to the fact that only you know where to draw the line. Just understand that the position of that line is usually never the same and changes from moment to moment;)