question was: "is it possible -in the context of writing a pd external
class- to invisibly patch some other object and attach to it/bind it?"
Frankly speaking, I have trouble understanding what the question was. Could you clarify, please? What do you mean by "attach to it/bind it" ?
Is your final goal to offer to a user who wants to write pd externals in tcl the option to create other pd objects and connect them from the tcl code of the external? Or was that only a part of a solution to another problem that could be solved differently?