Sorry for taking some time in answering, been away in the workshop for a week, and there is no internet there.
The only reason for filling the mast with foam is not having it filled with water when capsizing, there are no structural benefits what so ever, only the problem you mentioned.
I am not intending to inject the foam into the mast, but cut the foam to shape with a warm wire and put them inside before gluing the two halves together.
In the mast for the mini there will be no foam.
As for the weight I am targeting the same weight/lm as an alu spar of the same profile, and so far it looks good.
Will try to get up some pics on my page.