Won't the custom software install commands handle the unzip and unjar? Typically I would put any install commands relevant to each custom software in that object and use the blpackage to provide ordering and such.
In the BLPackage, I have a custom property called INSTALL_PATH which specifies where to install the software. As far as I can tell, the soft-linked assets don't have access to this property so their custom commands can not install to the location specified by this property.
if you want to use those custom properties, you'd need to 'hardlink' the packages into the blpackage - then you can modify the install commands of the depot software.
the problem is that the depot software will get laid down in a tmp directory on the target which the blpackage won't know about, so you can't use the external command, and the depot software can't see the custom properties if the depot software is symlinked.