There are alot more of those anti-sumping valves in use than you realise which give no trouble at all. The best ones are made for and supplied by Polly Palmer at Brie-Tie Motorcycles. Lst time I checked they were around £40.
Velocette fitted one as standard to all the big singles at the bottom of the oil tank. Only thing you needed to remember was to prime the feed line with oil before connecting it so you didnt have a big airlock. You would need to do the same if you use the type of Polly's that fits in a pipe (A50/A65) as opposed to the ones that fit onto the engine (A7/A10).
Hope this helps.