It should be possible to do in an extension, Arnaud, which is where I think it should go. I've been planning on trying to create one but haven't gotten to it yet. If you wanted to create and extension and send me the source I can add it to the standard liquibase extensions.
It looks like it may take a liquibase core pull request as well though, since AbstractChange.generateCheckSum() hard codes a call to "new ChangeLogSerialize()" vs. going through a ServiceLocator. Also individual change implementationscan can generate checksums however they feel they should which makes a generic case more difficult. If you wanted to take a stab at it and send a pull request and/or extension code I'd be very interested in seeing what it takes. Otherwise I'll get to it at some point, but it hasn't been a high enough priority yet.
Nathan