Installing bash on OpenSlug is simple enough:

ipkg install bash
This will install bash and any dependencies.

DO NOT log out. If you do, you won't be able to log in again.

Create /etc/shells, containing the following two lines:

/bin/sh
/bin/bash

I found I was unable to login after installing bash, even though I wasn't using bash as my default shell. Open up a new terminal, and try logging in a second time before logging out your original session ;)

[The last line should probably be /opt/bin/bash ; check to see where your bash is.]