Ctrl-O - why subshell only with bash, tcsh, zsh?

[Alexander Kriegisch]

MC FAQ:
> 6.6 When I use Ctrl-O I don't get a subshell.  How do I fix this?
> 
> Only bash, tcsh and zsh can be used as subshell.  Use one of those 
> shells as your default shell, and it will be used as subshell in GNU 
> Midnight Commander.

What is the reason for that? I use BusyBox ash on an embedded platform
(DSL/LAN/WLAN router) and would like to have a Ctrl-O subshell. What
would be technically required from a candidate shell in order to make it
work?
-- 
Alexander Kriegisch