Of course, that's what I do for her regardless of the situation as we live a full time Female Dominant marriage. Serving a lover, would be just an extension of my service to her.
However, most people would be reading a thread like this to see "the good parts" like fluffing, clean up etc., when, if fact, most service is really about much more mundane things like fixing drinks and snacks, turning down the bed, making sure there is lube and whatever other items they may wish to use ready and close by and staying attentive should they need your services and summon you from wherever you station yourself as you await their call.
If permitted to be present, helping by undressing them, folding and caring for their clothing, rearranging pillows, having warm damp wash cloths and towels readily available, applying lube for them so they needn't get their hands messy and in general, just being a convenience to provide for their needs so that they can relax and enjoy being with each other. Afterward, when they are ready, go warm the shower or run a bath if they prefer, set out soaps and shampoo and then go toss some towels in the dryer to warm and to present to them when they finish bathing. Also, change the bed linens so they will have a dry, comfortable bed to relax in.
Of course, one hopes to be allowed to actually be an active participant by performing the "usual" services such as clean up and/or fluffing, aiding with insertion or reinsertion, stimulating either one in a desired manner to add to their pleasure such as licking the wife's clitoris while she is being fucked or even being a third for DP if that is what is requested.
