Liquidation followed by a "Rangers 2012" is a mind boggling situation that will have the SPL constitution and rulebook being analysed and no doubt rewritten.
You are absolutely right that the SPL will have to find a way of admitting a "Rangers 2012" newco, and that it is an unprecedented opportunity to redress the balance of power.
A Rangers 2012, debt free and with a massive fan base and stadium, could be created within a few weeks but will need a league to play in. When they come begging to the SPL for a place, the SPL will comprise just 11 members, only 1 of whom is in favour of the current weighting of income distribution to the top 2 teams. So the wee teams could surely win a vote 10-1 - however, what does the SPL constitution actually say regarding the required voting majority? Currently 11 votes are needed to push through a change, but is there provision for that figure to reduce if there are less than 12 members?