I don't know if this is possible or not - it might just be a configuration file tweak - but would it be possible to slightly reduce the size of the Name and Comment fields in the calendar signup? Reason being, on mobile, the fields are "fixed length" and don't resize to take account of the screen. For me, on my phone, this puts the 'Sign' button under the text on the right-hand column, so I inevitably end up clicking on 'Members' or 'Stats' rather than the button itself.
Not a biggie at all as I can always wait until I have proper desktop access, it's only during working hours that I'm restricted to mobile.