Say we have a composite operation with 2 steps:

1) /extension=org.jboss.as.messaging:add
2) /subsystem=messaging:add

This will fail:

Failed to execute batch: JBAS014739: No handler for add at address
[("subsystem" => "messaging")]

This fails because at the time of validation the /subsystem=messaging:add is not valid.

To illustrate, the execution order is

Validate 1-2
1
2

A possible solution is to convert this to the following:

V1
1
V2 (works now because 1 has registered the subsystem API)
2

I think that should work but it's a very complex area, particularly in a managed domain, so it's not at all certain this would prove feasible.