Beginning with 7.x you can configure hairpinning on the ASA or PIX. 7.x does not run on the 501 or 506e.
The command "same-security-traffic" will allow you to do what you want, so if you can get an upgrade to an ASA you should be all set. Here are a couple hairpinning examples:
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www.cisco.com/en/US/prod ··· ae.shtmlFYI, the ASA5505 is the SOHO ASA.