After looking into Transmac and MacDrive, GParted and even more options, I did manage to solve the problem of formatting a partition into HFS+ so that it would be properly recognised by disk utility. I used diskpart, a command line program in windows.
All you have to do is create a partition using id=af. Eg. after starting diskpart (start, run, cmd, diskpart)
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create partition primary id=af
However, it would be in your interest to create the appropriate space for it beforehand. You do this by
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list disk
to see your disks, then
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select disk x
to select the disk no. x, then
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list partition
to see your partitions. At this point, if you already have the free space you require you can go straight to
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create partition primary id=af
However, if you want to delete a partition to make the space you need, you should type
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select partition x
to select the partition with no. x that you want to delete, then you can do
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detail partition
to make sure it's the right one, finally type
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delete partition
to completely remove it. Obviously everything on the partition will be deleted with it. Then you can type
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create partition primary id=af
Make sure you back up any important data first, and all that...