Well, much to my surprise it turns out an offline blogging tool that's compatible with Blogger.com actually does exist! It's called AvantBlog from DentedReality. Thanks to Vlad Campos (of PalmBR.com fame) and fellow PalmBloggers Yahoo Group member Sergio Lima for the tip!
Anyway, it's a simple concept: just add the AvantBlog page as a custom channel to your AvantGo account and HotSync. You must first log in once and then sync. Then, the Palm side is logged in when you next open AvantGo. Just start posting offline in AvantGo on the Palm side and the posts will appear after the next sync with the AvantGo servers.
I tried to get it working with iSilo, but had no luck as the two very necessary form fields on the AvantBlog page didn't render correctly. If you have any tips on this let me know.
So, it's back to my old PowerBook and Mac OS 9 to try with AvantGo...