Yeah, as specified this doesn't really take advantage of the design of polynomial models, which already accept each individual coeffecient as a separate, fittable parameter.  True, a different approach could be implemented that takes all the coefficients as a single parameter, but that's not how it works now.

Each coefficient of a 1D polynomial is, by default, given the parameter names `c0`, `c1`, ..., `c<degree -1>`.  For this it might be useful to define a B-spline model base type that also takes each knot as a parameter, perhaps like `t0`, `t1`, ..., `tm`.  Would it also be worth being explicit about the fact that this specifically implements a B-spline?