I am a ways off from this, too (2 year old) but it's an interesting idea.
The expected contribution for parental assets is up to 5.64%. So, in your example, $5,640 per year is at risk from the $100k. So, there is a potential benefit of 5.64% + 3% mortgage = 8.64% per year, for four years of college, and 3% thereafter. As a guaranteed payback (that is, if you would are not otherwise disqualified from assistance due to income, etc.) that seems pretty good. If all you had was $100k taxable, though, I would not think that is worth the reduction in liquidity. It also does not beat the return you could get from that $100k if it was invested in a stock mutual fund, particularly over the long term.