I am pretty certain that the only other way to have external links without using html tags is to just type in the complete url without any formatting. Just typing
http://tiddlywiki.com will be a link, but that isn't very useful.
Making a macro or widget like the link widget but that works for external links probably wouldn't be very difficult, but it would just be a wrapper on the a tag anyway, so I am not sure how useful it would be.
Do you think it would make a significant difference to be able to type something like
<$external-link to='http://tiddlywiki.com'>Some text or images here</$external-link>
instead of
<a href='http://tiddlywiki.com' target='_blank'>Some text or images here</a>
?
That is a sincere question, I have gotten used to just using the html and I am not sure if it would be helpful for someone to use one over the other.