it's fundamentally a VBox issue ( I wouldn't actually call it a bug per-se... ) ...
but the openwrt side of things is an interesting question... as even if virtualbox behaved better... there are similar instances where 'persistent adapter' assignment can be of use...
that said... automating such things is just as much a 'design' and 'philosophy' question as it is a 'practical need' based featureset...
i do see something 'official' happening in this space within the next 12 months... it's a good thing you've brought up one use case here ( you are the second user to post about this behavior here )...
but for now... i'd advise you to deal with it at the VBox level... specifically... the easiest thing being to allocate all nics prior to firstboot..