The problem in this scenario is that you expect the OpenWrt device to transparently switch roles from a 'dumb wireless extender' to 'router/ DHCPd in charge'. This can't really work without some kind of manual decision, as multiple DHCPds in one network create a havoc for your devices - and because 'the other device doesn't seem to answer over wireless in time' is not really a reliable enough metric for this decision (you'd end up with split, but competing network segments before long). The devices simply can't magically read your mind - and all clean layered approaches I can come up with would require even more power draw.
The easiest approach would probably to kill this problem with hardware... One OpenWrt device configured as router/ DHCPd, one as extender - using a mutually exclusive electrical switch for ISP-router+OpenWrt-extender or just OpenWrt-DHCPd. I'm aware that this is probably not possible (for the distance between them alone), but maybe it jumpstarts some ideas.