Hi, Rene,

Placella et al. used a method originally introduced by Griffiths et al.  with the modifications of DeAngelis et al. , It works well for soil. you may try this method.

Placella et al., Rainfall-induced carbon dioxide pulses result from

sequential resuscitation of phylogenetically

clustered microbial groups. PNAS, 2012,109:10931-10936.

I think the liquid-N2 preservation is fine. Sometimes it works better than RNAlater. But I wondered how easy/hard when you have to wash the biofilm from the frozen swabs? It's been more than a month since you posted this question. I am curious about the updates. 

Cheers.