This feature can easily be provided using the exsting external wiki references (see FormattingRules) approach.

E.g. To link to Amazon.com for ISBN codes add the following in the relevant topic or in a topic called ExternalWikis:

 @Amazon=http://www.amazon.com/o/ASIN/$$$

Then you can use:

 <ISBN>@Amazon

The $$$ in the definition URL will be replaced with the ISBN provided. Such as 078214327X

Alternative web sites can be used, you just need to modify the external wiki reference in ExternalWikis accordingly.