I don't know well WHFC, but I think that certain problems can occur.
SMERPFax doesn't modify Hylafax, and doesn't change the way it works, but the mode of configuration is different.
Indeed, several files of configuration are modified by templates and inside panel.
It implies that the modifications brought by the users will not be saved into configuration files after a restart of the server, because in that case, these files will be dynamically generated by templates.
Except this problem, I think that the hylafax clients work normally with the contrib SMERPFax.
Let's try & enjoy...
