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The open-source, DRM-free Open Printer shows off a working prototype
(www.techspot.com)
This is a most excellent place for technology news and articles.
From what I can tell the print head is built into the cartridge and that's the hard part for printer technology.
But I think you might be right depending only on the hp cartridge will open them up to cease and desist. If they make it also use a cannon cartridge that also has a print head then I think they might be ok