Dunc,
You can do the belt change without the tensioner tool as the important check to make is that the projection of the tensioner mechanism is 3,8 - 4,5 mm (I expect you already have this info). When I did mine (EVO 1) I started with the pin in the tensioner and after removing it (the pin that is) slightly slackened/moved the pulley gradually until I had about 4mm projection (you can get verniers in to check this without any problems). The only problem with not having the tool is if you go outside the mechanism spec as you need to remove the tensioner, stick it in a vice and re pin it! Never considered the circlip plier route but may be a good way of doing it. Remember to let the belt settle for about 15 minutes and then re check the tensioner protrusion.
Must admit that I don't like the hydraulic tensioner set up - I prefer the locked tensioner pulley arrangements more.
Do you have a work instruction for doing the belt change - as I might be able to help you there.
Regards
Mark