Will I be able to (ab)use a FIDO U2F compliant hardware token to decrypt data?
No, you will not be ble to abuse challenges in that way. You may be thinking about e.g. raw RSA, where signing and decryption are mathematically similar. However, this is not possible in FIDO U2F:


All the defined signature algorithms use SHA256 on the signed data, meaning that you'd have to brute force search for something that produces the correct signature.
Not all the hashed/signed data is controlled or even necessarily known by the server. E.g. the signer has an incrementing counter that's part of the signed data.