Is there any restriction on migration to sites still in beta?
If not, why not add the option to migrate questions to http://robotics.stackexchange.com?
This question seems to be much more focused on mechanical than electrical, for example.
I'm not sure about private betas, but moderators can migrate questions to any Stack Exchange site, including those in beta. However, this is a manual process. If you see a good question that may belong on another site, go ahead and flag the question and let us (the moderators) know.
Based on the number of questions migrated to other sites, it doesn't make sense at this time to add an option to the off-topic close dialogue for any other sites by default.
Option to migrate depends on the frequency of the target site, and the mods. If op needs to migrate, or you/other users think it should be migrated to SE x or y, you can either close with "off topic -> other" and add a comment, or flag the post as other (needs mod attention), or both, if the question requires it.
I too think that we should have an option to migrate to DIY or maybe Auto SEs