libbitcoinkernel public api define btck_ChainType as a single byte. As such, there's no way to inform the signetchallenge parameters so that the resulting btck_ChainstateManager can work with signets other than the "public" one.
Also, as far as I can understand, none of the functions exported in the public API can be used to achieve such a result. This information comes from ArgsMann in the node, which is abstracted away in the kernel library.
I wonder what would be the best approach to solve this. Maybe a richer btck_ChainType capable of carrying more information.
Found this with @oleonardolima during a @vinteumorg bitcoinkernel study session. We were trying to load this datadir, a custom chain we use in the educational programs.