If you take a look how it is done in

xschem

it is ultra simple. It does not include the callbacks, which in

xschem

can be implemented as TCL functions.

format

and

lvs_format

(spice, cdl respectively) define the netilist entries used by

xschem

to eleborate the final netlist. The template section define the device itself with the default values of the parameters.

lvs_format="@spiceprefix@name @pinlist @model l=@l w=@w ng=@ng m=@m"
format="@spiceprefix@name @pinlist @model w=@w l=@l ng=@ng m=@m"
template="name=M1
l=0.45u
w=1.0u
ng=1
m=1
model=sg13_lv_nmos
spiceprefix=X"

In

xschem

also exists a possibility to implement, in TCL, some ERC rules to check the validity of the parameters but I believe this check can be part of the callback functions implementation.