PenDriveLinux.com meets their (GPL) obligations with integrity. The latest sources are certainly available.
Just follow the Credits, Resources and Sources
link at the bottom)
There's also source for an obsolete MultiBootISOs tool there, but it's based on grub4dOS.
It's useful to study the directory structure and configuration files used, since for many distros it's mostly
format the stick (YUMI uses FAT32),
make it bootable (with syslinux),
extract most of the iso (7zip), and
add the startup cfg file(s), usually with adjusted boot parameters (cheatcodes).
The sourceforge project multiBootUSB was updated 2013/01/29 ... though it uses grub4dOS too.
Note: Using a dd-based tool to "dump" an image onto a flash device
destroys any prior file-system or partitioning
overwrites the target partition; access to any data not destroyed will be lost, and capacity will be reduced to the image size. Does that sound like efficient use of flash storage?
Or open for multiboot or customization? Overwriting a partition is not