All my research lately said that 512MB devices (e.g. Lumia 520) could never go past 10586. I don't remember how exactly but I have a Lumia 520 which somehow got stuck at 14393.1066. Until today I had no luck with RS2 builds but the recent announcement by Microsoft that "unsupported devices can use RS2 at own risk" or whatever it actually said got me to thinking I should try again.
A quick browse of the Microsoft Update Catalog searching "10.0.15063.251" shows three updates available: for NTN-FI, ELI-FI and 000-FI based devices. No mention there of specific model number variants at all but drilling down the details shows requirement of "Windows Mobile 10 Production: Approved RS2 Device Targeting" and (OSVersion >= 10.0.14393.0 or < 10.0.15063.251) so I decided to try one of those.
I already have the Interop Tools registry browser on the phone from previous attempts.
In HKLM\System\Platform\DeviceTargetingInfo I made the following changes:
PhoneHardwareVariant = RM-1085
PhoneManufacturer = MicrosoftMDG
PhoneManufacturerModelName = RM-1085_11302
PhoneMobileOperatorName = 000-FI
PhoneModelName = Lumia 950 XL
Then in Windows Insider settings I selected Release Preview.
Behold, my 512MB Lumia 520 has downloaded "Windows 10 Technical Preview for phones (10.0.15063.251)" and "Insider Release Preview configuration update" and is currently preparing to install. I'll post back later after it either fails or succeeds.
Side note: during the time I've had 14393 on this phone it has been very sluggish and many apps crash randomly. I suspect this is because of the tiny memory available.