Sony Ericsson :: W800i - Which Tools Would Be Best To Use?

As a spare time J2ME developer I sometimes run into the annoying security issues, that makes messages popup, and requires my application to be signed if I want to avoid those popups. A certificate costs about $300-$500 a year, which is a lot for a spare time developer who's not making money on it, but mostly is just developing applications to myself. The alternative solution is to put my own root certificate on my phone. In order to do this I need one of the tools that lets me hack my firmware, as far as I understand.

My question is, which one of those tools would be best for me to use? Which one lets me add my own certificate to e.g. a W800 or W760 relatively simple (and preferably free)? Back in the old days I tried Far Manager to give my W800 another set of icons, and a boot-up splash image. I remember nothing about it though, except that it looked like a DOS program, and I bet there are simpler ways to do it today. I've also tried DaVinci very briefly once I needed a free service - but needed to pay anyway to get access to that free service. So which tool would you recommend me, if all I need to do is to give my phone an additional Java Certificate?