Can I sign up to a Windows 10 free upgrade now and defer the actual install until after the July 29 deadline?

I'm currently somewhere with poor quality internet connections, working on some time-sensitive projects. I'd like to update from Windows 8.1 to Windows 10 at some point, but not for a month or so until I'm somewhere with better internet connections, with more time to fix any bugs, glitches or compatibility issues.

Is this possible? Is there any way I can sign up for the free install now before the deadline, but defer performing the install until later when I've got the time and facilities to make sure it happens smoothly?

No.

That is because the licensing component of the free upgrade is a digital entitlement created by Microsoft Activation Technology. "Signing up to upgrade" does not give you this digital entitlement. This is deliberate: Microsoft wants people to move on to Windows 10, not to promise moving on and never doing so.