Wishing Linux had partitionable loopback/virtual disk

I wish Linux had a loopback (stored in a file) or virtual (stored in RAM) disk block device which supported partitions. I know some of you will be jumping the reply button to tell me about the offset option for loopback. Well, no, that isn't it. I want such a device for the purpose of running bootloader installs on. That requires the bootloader to be able to access both the whole device and the partition(s) on what are whole/part related device inodes.

For now the workaround is temporary USB memory sticks. They are just slow.