opkg, unlike rpm and dpkg, doesn't let the package being upgraded run scripts. As an example of where this is a problem see #4213.
This isn't going to be fixable for 1.4, and we need to discuss with upstream if this is something that will be fixed at all.
Not a massive issue and opkg has a new maintainer. Sent upstream as https://code.google.com/p/opkg/issues/detail?id=104, closing this as WONTFIX.