Ok - I've been thinking about this for the last couple of hours.  It's not a bad idea; but it needs some improvements:

1)  This should be optional (and off by default).  It also needs to be documented.
2)  This should use a different set of options to inbound IMO as you're very likely to want to have a different set of rules for outbound because:
   - SPF_TEMPERROR and SPF_PERMERROR would be undesirable (DENYSOFT on AUTH clients is nasty).
   - It would be nice to be able to reject outbound mail where result is SPF_NEUTRAL (possibly), SPF_SOFTFAIL or SPF_FAIL.
3)  The external IP mechanisms are rather fragile (what happens if a host has multiple external IP addresses - how do we know which to use?).   See my later comments.