API for specifying customized transforms in ChiselStage (and dependencies?)

[poemonsense]

Type of issue: Feature Request

Is your feature request related to a problem? Please describe.

We have some (lightweight) customized transforms before emitting the FIRRTL. They are still written in Scala. Currently we have to build our own stage with the transform in ChiselStage preserved and customized transform inserted.

Is there any clear API for the users to inject some transforms into the stage? The difficult thing may be specifiying the dependency.

Describe the solution you'd like

Allowing customized transforms only just after Convert? Then the dependency issue does not exist.

Describe alternatives you've considered

We are copying the code of ChiselStage (the transforms) and build our own Stage.

Additional context
Include detailed explanation, stacktraces, related issues, suggestions how to fix, links for us to have context, eg. Stack Overflow, gitter, Scastie.

What is the use case for implementing this feature?

Customized transforms to collect information before emitting FIRRTL.

[sequencer]

Maybe I need to upstream this:
https://github.com/chipsalliance/t1/blob/real-rocket-v/elaborator/src/Elaborator.scala
and its usage
https://github.com/chipsalliance/t1/blob/real-rocket-v/elaborator/src/rocketv/ALU.scala
for migrate out from Stage API.
wdyt @jackkoenig?