I've spent several hours checking all Indymedia sites linked from the front page to see if they accept encrypted URLs, and here is the resulting ruleset for use with HTTPS Everywhere. Let me know if you find something that isn't covered by this, or that doesn't work.

<ruleset name="Indymedia">

<!-- the main indymedia.org and indymedia.org.uk domains -->
<rule from="^http://(www\.)?indymedia\.org(\.uk)?" to=" https://www.indymedia.org$2"/>
<!-- london.indymedia.org.uk is different to indymedia.org.uk/en/regions/london/ -->
<rule from="^http://(www\.)?london\.indymedia\.org\.uk" to=" https://london.indymedia.org.uk"/>
<!-- Nottingham has its own setup -->
<rule from="^http://(www\.)?nott(ingham|s)\.indymedia\.org\.uk" to=" https://nottingham.indymedia.org.uk"/>
<!-- Bristol has its own setup under the indymedia.org domain -->
<rule from="^http://(www\.)?bristol\.indymedia\.org" to=" https://bristol.indymedia.org"/>
<!-- All other UK regions are in region subdirectories of the main indymedia.org.uk domain -->
<rule from="^http://(www\.)?(birmingham|cambridge|liverpool|manchester|oxford|scotland|sheffield|southcoast|world)\.indymedia\.org\.uk"
to=" https://www.indymedia.org.uk/en/regions/$2"/>
<!-- Scotland has its own domain as well now, indymediascotland.org, but that doesn't have an encrypted URL -->
<!-- Northern Indymedia has its own domain -->
<rule from="^http://(www\.)?(northern-|northern.)indymedia\.org" to=" https://northern.indymedia.org"/>

<!-- The print and satellite projects aren't encrypted, but radio and video are -->
<rule from="^http://(www\.)?(radio|video)\.indymedia\.org" to="https://$2.indymedia.org"/>

<!-- Some international Indymedias have encrypted sites, others don't.
Many of them have self-signed or invalid certificates, so your browser might complain, but you can override it. -->

<!-- Africa -->
<rule from="^http://(www\.)?(estrecho|kenya|southafrica)\.indymedia\.org" to="https://$2.indymedia.org"/>

<!-- Canada -->
<rule from="^http://(www\.)?(maritimes|bc)\.indymedia\.org" to="https://$2.indymedia.org"/>

<!-- East Asia -->
<rule from="^http://(www\.)?(qc)\.indymedia\.org" to="https://$2.indymedia.org"/>
<!-- Europe -->
<rule from="^http://(www\.)?(abruzzo|athens|austria|barcelona|belarus|bxl|brussels|calabria|emiliaromagna|euskalherria|grenoble|istanbul|italy|linksunten|madrid|malta|nantes|ovl|paris|piemonte|roma|switzerland|torun|toscana|ukraine)\.indymedia\.org"
to="https://$2.indymedia.org"/>
<rule from="^http://(www\.)?indymedia.ie" to=" https://www.indymedia.ie"/>
<rule from="^http://(www\.)?indymedia.nl" to=" https://www.indymedia.nl"/>

<!-- Latin America -->
<rule from="^http://(www\.)?(argentina|bolivia|sucre|colombia|ecuador|peru|qollasuyu|rosario|santiago|uruguay|venezuela)\.indymedia\.org"
to="https://$2.indymedia.org"/>

<!-- Oceania -->
<!-- nothing yet -->

<!-- South Asia -->
<!-- nothing yet -->

<!-- United States -->
<rule from="^http://(www\.)?(arizona|hawaii|houston|sandiego|seattle|)\.indymedia\.org"
to="https://$2.indymedia.org"/>
<rule from="^http://(www\.)?.indybay\.org" to=" https://www.indybay.org"/>

<!-- West Asia -->
<rule from="^http://(www\.)?(israel)\.indymedia\.org" to="https://$2.indymedia.org"/>

<!-- Topics -->
<rule from="^http://(www\.)?(biotech)\.indymedia\.org" to="https://$2.indymedia.org"/>

<!-- Process -->
<!-- Ironically, tech.indymedia.org doesn't have a https site -->
<rule from="^http://(www\.)?(lists|docs)\.indymedia\.org" to="https://$2.indymedia.org"/>

</ruleset>

Nice one anon - works for me.

I put the script on my blog here so that people can see and copy it without the space that gets added when this site turns URLs into like (like this to=" https://london.indymedia.org.uk )

Here's the script without spaces ...
 http://penguin.ox4.org/node/397

When fully tested it, I think it's worth sending to EFF to include as a standard part of HTTP Everywhere.