The answer (as you suspected) is no. There is currently no provision in bind to do what you're asking. If your internal server responds as authoritative for a zone, it assumes it has all knowledge of that zone.
That said, it might be worth attempting to delegate the external subdomains using NS records.
internal zone:
$ORIGIN bar.com.
$TTL 14400 ; 4 hours
bar.com. IN SOA ns1-int.bar.com. admin.bar.com. (
2012020206 ; serial
86400 ;refresh (1 day)
600 ;retry (10 minutes)
1814400 ;expire (3 weeks)
60 ;minimum (1 minute)
)
IN NS ns1-int.bar.com.
IN NS ns2-int.bar.com.
; define internal nameservers
ns1-int.bar.com. IN A 10.0.1.1
ns2-int.bar.com. IN A 10.0.1.2
; internal records
www.bar.com. IN A 10.0.0.1
db.bar.com. IN A 10.0.0.2
proxy.bar.com. IN A 10.0.0.3
; define external nameservers
ns1.bar.com. IN A 2.3.4.5
ns2.bar.com. IN A 2.3.5.6
; delegate subdomain foo to external nameservers
foo.bar.com. IN NS ns1.bar.com.
foo.bar.com. IN NS ns2.bar.com.
At this point, your internal dns server (assuming it allows recursion for localnets) will go to ns1.bar.com if it receives a request for foo.bar.com.
Of course, you have to do this for every subdomain you want to refer to the outside servers, but after the initial setup for each external subdomain you will not have to mess with the internal zone anymore whenever changes are made to the external subdomain records.