My guess is they'll insist the format is 'regional'. Otherwise, they'd have to have the DUP on as often as the Lib Dems, and if they do that, the the other NI parties would demand a right to appear against the DUP, and the whole thing would be unworkable.
I expect they'll think the fact they occasionally have SNP people on when hosted in England will be enough to say the SNP are punching above their weight.
An alternative could be to have the programme come from Scotland more often, allowing SNP more airtime than they'd have by population / vote share alone, but to pacify the rUK, in each Scottish edition, the issues discussed would still be largely UK relevant, ad SNP would still be safely outnumbered by unionists.
My guess is that the BBC will do what they fukkin want..