This would require an API for Diaspora which do not exist yet. You can search Loomio and Github issues for more information about the implementation of the API.
Your idea has been suggested to the Loomio developers before, so they’re already aware of it as an idea. As Erwan says, Diaspora would need a working API for this to be possible.
@juansantiago: Once OpenID Connect is integrated into diaspora, every plateform will be able to propose a sign-in with diaspora without asking any permission.
The good point is that, the information that will be exchange in the OIDC flow will be very limited (there will not even be the email). So, this is actually no big deal to propose that feature.