I can think of at least two ways you could do this: one that uses only the dialplan and one that uses a script. If you don't mind using a scripting language then you can make it very clean:

<action application="set" data="api_hangup_hook=luarun chancleanup.lua ${hangup_cause}"/>

Then have your Lua script handle all the if-then-else or case stuff.

Question: are you trying to transfer the a-leg to some other destination if the b-leg hangup is a specific cause, or are you just doing some external cleanup stuff? Just curious...

-MC