I've been on the lookout for a proper tool of this kind for years. It makes writing music to a completely different and a much more versatile experience.
No more buying or printing out blank lines - now writing music is as simple as writing a note on the backside of a piece of scrap paper.
The tool fits in any pocket and can contain ink for quite a number of pages. It requires a bit of practice, but after testing on a couple of pages I have been able to make quite good consistent lines. You will need a proper plane surface for the lines to look good.
A small note: Use stamp ink, NOT writing ink for this tool. I use Colop Premium Stamp Pad Ink 801, which does not dry out on the pad but still appears dry immediately after being applied to the page.