What if the Taxpayer cannot access the equity in their assets? Can it be excluded from the Offer calculation?

No. Unlike a currently-not-collectible determination, the IRS considers the equity when calculating the RCP for an Offer. The fact that the taxpayer cannot access the equity is not the concern of the IRS.
The way to look at this is that the IRS is telling the taxpayer the number it is willing to accept to settle the debt. The taxpayer, if they accept that number, can get it from wherever they want. If the taxpayer cannot raise the money than they cannot do the Offer.