I'm not a lawyer, but from my legal understanding, they can't just sue you up front. Worst case scenario, they'd send a cease and desist, which basically means "stop what you're doing or we will
sue you". Pretending that the Lego Group somehow notices a mod for a freeware game as small as Glest, I'm sure they wouldn't even go that far.
Based on the Lego Group's page about fair play
, just don't use their logo and include a disclaimer somewhere that the Lego group does not endorse your product. Since you're creating a free product that does not in any way hamper the Lego group's ability to sell their product, I do believe the usage also falls under the protection of fair use
Also, the Lego Group hasn't raged at elim's above animation yet, so, yeah, I'd say you're safe. Personally, I'd say go ahead and call it Lego. I do believe there already is a "block faction".