Hugh,
A subject dear to my heart. I have used small APO astro refractors for birding for the last 20 years because of their optical superiority to birding scopes. Of course there was a much bigger difference in the 80's than now.
A Pentax 80mm EDscope I recently tested did have a significant problem with vignette coming from the porro prism housing. In this scope one part of the prism housing moves to change focus. At close distances starting at about 25m the prism housing begins to vignette the objective until only about 68mm of the objective is actually in use at closest focus.
Some of the eyepieces made for birding scopes are actually just as good as the best astro eyepieces. The wide field Swarovski and Nikon eyepieces are some of my favorites, and the Zeiss and Swarovski zooms are far better than the zoom TeleVue markets. Any of these can be adapted for use in your scope. The Zeiss zoom would produce 19-57X and the Swarovski 21-63X. Zeiss makes very expensive adapters and less expensive ones for Zeiss, Leica and Nikon eyepieces are available from APM Telescopes in Germany (their Zeiss 2" adapter can be used for the Swaro zoom by substituting longer hex screws).
The main problems I have using this type of scope for birding are weight, the need to be careful that the diagonal and eyepieces don't loosen and fall off when I'm moving around and the lack of waterproofing. But, you can expect queues to form behind your scope at good scoping places as word spreads that "the bird really looks good in that big odd scope over there".
Henry