Agreed that FPC has the capabilities in the general case (although I assume not for devices which use that specific loader), but I thought it worth mentioning anyway.
MarkMLl
The way AVR support is set up it can... for several of the family.
I am no real expert on AVR's (I am a tinkerer, tend to blow up things) , but people on this forum are (Jeppe?). So you might ask them. They are professionals on the subject.
AVR family users - on this forum - are usually either highly skilled amateurs or professionals.
But anyway: if you provide links to the third-party sourcecode it is highly likely I can convert it quickly, but just for Windows. Cross-platform support is another issue.