I haven't found anything to merge line types, at least so far.
Crank, I'd be interested in seeing what you use to merge text styles. With all the custom styles that just happen to have identical names, and with the users who "make their lives easy by setting the STANDARD style to their preferred font" merging text styles could be problematic.
As Tim noted in thread 17496 his code does not merge styles and it leaves behind the styles (and associated layers) that could could not be renamed. irneb's code from AUGI uses LAYMRG. LAYMRG does not leave behind any layers with $0$, but it does leave styles with $0$.
The code for LAYMRG was clear text back around 2004 (and can be found on Google, file name lydelmrg.lsp). It might be interesting to try to modernize that code.