Hi Andy,
No, I am not aware of any other email providers that allow you to attach files to away messages.
I figure one of the driving reasons for this not being a feature with many email hosts is the denial of service potential.
Allowing attachments on away messages would allow a malicious email senders to bombard your account with many small emails and cause your mail host to use large amounts of bandwidth to send the emails, and the recipients would be getting hammered with the large emails too.
Gmail has great spam filtering and other anti-abuse mechanisms so maybe that's why they can allow the feature while many other providers can not.
It might not be the answer you are looking for but hopefully helps to explain why it's not a common feature.
If you do find someone else who allows you to attach files to your out of office / away messages please reply so everyone else can benefit from the answer.

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