Can an U2F public/private key-pair for signatures be used for encryption?
No.

It is possible to derive keys from deterministic signatures (or non-deterministic signatures that can be converted into a deterministic value), for example Android uses Trusted Execution Environment's RSA signing plus scrypt to bind Full Disk Encryption keys to a specific device.

However, ECDSA signatures are not deterministic (there's RFC6979, which specifies deterministic use, but U2F doesn't use it), so each generated signature will be different for the same message and key, thus you cannot derive the same key from multiple signings.