hi lynx,
welcome to the forum. you do not need to edit anything to have google spider your lo fi version since there is a link to it from your forum homepage (at the bottom of each page). the spider should follow that link and then proceed to spider the lo fi forum. hopefully the spider will find both the lo fi version and the full version. i find on some of my forums that about half of the visitors arrive via the lo fi version.
to make it easier for the google spider to find your lo fi version you might include additional links on other pages or maybe include a link in your forum signature. i personally have not done this but it might be worth a try.
one thing i would try is to do a
site:domain.com search in google and see how many forum pages are actually spidered. if you find a high percentage of pages are included then you might not need to worry too much about the lo fi version since people are still finding your site.
i have heard a few people on the IPB forum trying to repilicate the lo fi version as their default skin. This might be what you mean about people editing the php files.
one thing that i think would be useful (if i can figure out how to do it) is to add a rel="nofollow" to each href link in the forum that leads to a non-important page...e.g. the links at the top of the forum that lead to "Help", "Search", "Members", "Calendar". The spiders often follow these links and it is totally pointless. I will have a think about how to do this.
best of luck,
cheers, glen