This is exactly what I'm seeing and am still investigating. I don't want to get your hopes up though, as I think you're out of luck, unfortunately. I might be too, but I'm in with a chance and here is why.
I installed my Insider copy right at the start of the program on VMWare Player where it activated with my Live account. It also upgraded to the RTM copy when it was released and is activated. So far so good.
I then downloaded the ISO using Microsoft's tool and installed a clean copy on my laptop, again logging in with my Live account, but it won't activate, just like you've seen. Reason given is that the key is blocked. However, what I'm now gonna do is image that VM onto the laptop's HDD and see if that works. It may remain activated (doubtful), require a phone activation or simply not activate at all. Let's hope it activates.
BTW, I never opted out of the insider program and am still on the Fast updates, so my OS version is currently showing as 10.0.10532, which is later than the 10.0.10240 RTM build - it did a full OS install for it.
Finally, even though you can't activate yours, you can still do the upgrade thing from W7 or W8/8.1 so you're not actually stuck, you just don't get the Insider version.
