RetroLoader v0.5 by Pastbytes (released August, 2021) - Very cool idea / program.  Following is the description by the author:

RetroLoader is a browser for Commodore 128 capable of loading and running Commodore 64 programs directly from 128 mode. At the moment it only supports .prg file loading and directory browsing, and includes JiffyDOS compatible fast loading to speed up loading in SD2IEC. If the drive is a 1541 without JiffyDOS, the program will not detect the drive and will give the option to disable JiffySoft before retrying to read the directory.

Here's a link to the authors website with more information about the program.

Here's a link to a Lemon64 forum thread about RetroLoader

How the program works:

• RetroLoader is a browser and loader for Commodore 128 that allows you to load and run Commodore 64 programs from 128 mode.
• The program checks if the current drive is an SD2IEC, and if so enables directory browsing.
• Only PRG can be loaded, there is no support yet for images (D64, D71, D81, DNP).
• RetroLoader uses JiffySoft 128 (SJ128) by Robert Willie:  https://sites.google.com/site/h2obsession/CBM/C128/JiffySoft128
• The loading of the selected program occurs in mode 128, which is why it takes advantage of the fast loading with drives 1571 and 1581.
• 62K of the second bank of RAM is used to store the list of files / directories, so it can handle a maximum of 3968 items in a directory.
• Once a PRG is selected, it is loaded to bank 1 in the same direction as it would be in mode 64. This avoids conflicts due to the differences between the memory maps of the modes 64 and 128. The maximum size that a PRG can have is of almost 62K, since the load is done in a memory bank where there is only RAM, so there are no conflicts with ROMs and I / O.
• Once the PRG is loaded, part of the code is copied to the 64-mode cassette buffer ($ 033C). This is a point of no return to mode 128, since it destroys the system variables, due to the differences between the memory maps.
• The code in $ 033C copies the PRG from bank 1 to bank 0 (overwriting the browser code), configures the MMU to go to 64 mode without transferring control to the system, configures the system variables to "simulate" that we just finished. loading a program with LOAD on a freshly initialized C64, loads 'RUN' + RETURN into the keyboard buffer, and transfers control to BASIC 2.0.
• RetroLoader can load any PRG that falls between the addresses $ 044B and $ FC00.
• Known bugs: Do not check that there are no more than 3968 items in a directory, it is recommended not to exceed that number. There is some incompatibility between JiffySoft and a 1541 drive that makes the browser not detect the drive, in this case it is recommended to disable JiffySoft and retry reading the directory.