In absence of any direction from the owners manual, it appears that what you don't want to do is have too little oil. Since the oil tank is the "sump" tank reservoir (since it is a dry sump engine) for the oil to collect to...I would think along the same line of thought of bigern 2000 that it could be as full as to just under the oil cap opening since all the oil is doing from this tank is gravity feeding to the engine. Don't the Harley engines feed the engine the same way...with an oil tank, I believe they are dry sumps also?
Unless there is a vent from this tank back to the engine I do not see what harm there would be to this practice. I called my shop (didn't talk to Dink the owner and chief mechanic) and the guy I talked to, when I explained my "problem", just said to fill it up to the maximum mark on the dipstick but he didn't really answer my question. I think I'll ask Dink personally when I next visit the shop, and see what he has to say.
Dennis