I have no plans at this point to support that feature on Mono.
I've been working with Mono since 2004 and while I like Mono and the idea of running mojoPortal on linux, it has always been a buggy platform that I would not recommend for any important production deployment of mojoPortal. I would say that any site that wants to do ecommerce is an important production site and I would not recommend running such a site on Mono at this time therefore I don't plan to put any effort into testing or supporting WebStore on Mono. My current recommendation is if you want to do ecommerce then use Windows hosting. I would love to be able to recommend Mono for production and since 2004 I've thought some day I will be able to recommend it, but so far it is always someday in the future and I'm still waiting. Each release of Mono has improvements but often also new bugs. I don't really blame the Mono team because they try hard but they are chasing a moving target as ASP.NET keeps evolving with new versions and Mono will always be a few steps behind or more. When someday comes and Mono is a stable and reliable platform for mojoPortal in general, then I would consider trying to support WebStore on Mono, but not until then.
Best,
Joe