Why not do this as a regular WP premium theme?

As far as I can see, your theme's unique feature is that posts are attached to specific locations on the map. I understand that your original client runs a blog network based upon WPMU, but this functionality has no inherently MU-specific requirements.

By requiring WPMU, you are massively cutting down your potential market. Far more people are willing to install regular WP than WPMU, why insist on WPMU when your customers might not want or need to run a blog network?

A quick rewrite for WP compatibility would result in far more sales.