Actually, you can estimate the active surface area using other porosimetry techniques than nitrogen sorption, for example, mercuroporosimetry or water intrusion. You can make acquaintance with their application for MOFs, for example, using such articles as:

PCCP communications 7, (2015), 1572-174 

Nanoscale 7, (2015), 8803-8810

If you know the crystal structure, you can calculate it with a Voronoi decomposition, using the Zeo++ program for example.

Besides the good answers stated above, you do not necessarily need to use Nitrogen as your adsorbing gas, you may in fact need to use Argon or even CO2 can be used for surface area calculations. Another thing to keep in mind is that some MOFs are not stable without the presence of solvent at the unsaturated metal sites, so it may be of some use for you to look into how researchers working with those materials estimate surface area measurements. Many use computational methods, or mercury porosity measurement.