Opensearch plugin: Use %20 instead of + space encoding

Question

Is there a way to write an open search plugin to encode a space character into %20 instead of into + character? I.e. the lala plugin -- http://mycroft.mozdev.org/search-engines.html?name=lala -- contains the following line:

<os:Url type="text/html" method="GET" template="http://www.lala.com/#search/{searchTerms}"&gt;

Searching for Tom Waits results in the url ...#search/tom+waits instead of .../#search/tom%20waits