When wic creates an image the bootloader configuration is created based on options in the .wks file. If there is need for a very specific configuration, these options are not enough and the configuration file needs to be modified/replaced after the image is created. Examples of these specific needs would be multi-boot system or scripting.
I'd like to propose alternative solution for this problem - to prepare boot rootfs by bitbake and use wic source plugin to convert it into the partition. you can see similar example in meta-selftest/recipes-test/images/wic-image-minimal.wks This way bootloader configuration of any complexity could be handled easily. There is one limitation of this approach - it works only with EFI booting. I hope it's not your case though.
For EFI is a good option to use a recipe to create the boot partition, however the requirements here is to have something that can also work with MBR. Hence the bug. I sent the patches to the ML. http://lists.openembedded.org/pipermail/openembedded-core/2015-December/113581.html
This bug has been solved by commits: http://git.yoctoproject.org/cgit/cgit.cgi/poky/commit/?id=9773faadfeee0e64fc405e0192f32aa8eca95a76 http://git.yoctoproject.org/cgit/cgit.cgi/poky/commit/?id=f95f729518296177a6280a6e19a0c1433d499710 http://git.yoctoproject.org/cgit/cgit.cgi/poky/commit/?id=70338732ac7d6cf8a012e6861c316bcbd66e1328 http://git.yoctoproject.org/cgit/cgit.cgi/poky/commit/?id=c59dc3bae09ccb3749539dd5f1ddc12e26591523 http://git.yoctoproject.org/cgit/cgit.cgi/poky/commit/?id=8612f260048fe53f46149708baa6bd1985a4eb9f http://git.yoctoproject.org/cgit/cgit.cgi/poky/commit/?id=d7baeb5fad2630986556d4af38a2e2c67f7250c2