The parts you left out did not talk about reader conditionalization
in ASD(F) files. So I'm confused by what you mean exactly.

Sorry, I understood the reader macros were intended to appear in the ASDF itself. I obviously misread your email, but I needed a third reading to grasp it all -- please tell me whether the following description is right :-)

Goal:
Library B has some facilities that make sense when library A is present. The goal is to make these facilities compiled and loaded whenever possible.

Solutions offered:

1) Let the library B :weakly-depends-on A and use the fact that library A introduces a new feature. The code in library B is then populated with #+/#- depending on that feature.

2) One of the components in the system definition includes all the code that depends on A and is loaded only when that :feature is active

3) One of the components in the system definition isolates all dependency on A and this component :weakly-depends-on

4) Make two separate system definitions, one with and one without code for A

I believe 1 and 2 have problems because as you say the feature might be present in the core. #3 is better because the code that depends on A is only loaded when ASDF knows that A is present and loadable -- no dependency on non-standard features. Method #3, if working (which Fare seems not to be sure about) is also the substitute for reader macros in ASDF files.