Most people implementing ECC signatures are going to end up only handling a chosen group of named curves whereupon there aren't any curve parameters to check.
For example in Signal IIRC they use Curve25519. So there isn't any code about parsing or checking parameters, it's just here's some code that implements Curve25519.
NSS (the library used in Firefox on every platform) only accepts NIST's P-256 and P-384 named curves. It is parsing an ASN.1 data structure which can have parameters for curves, but if parameters are present instead of a name NSS's code gives up immediately because it's only looking for those specific names. (These aren't actually human readable names, they are OIDs such as 1.2.840.10045.3.1.7 for curve P-256)
By the way, when you say closed src allowed... do other libs in the opensrc space check the curve Params?