Especially tailored for the new custom firmware by M33 (a.k.a. Dark AleX), Custom Firmware Extender at version 1.6 is almost at the point of full functionality. Almost. Homebrew developer Cpasjuste took the time to make the plugin set work "almost normally" for M33's 3.71 CFW and it may exhibit some limitations in connectivity.

Like Custom Firmware Extender version 1.5, version 1.6 will allow you to stream ISOs via USB from your computer's secondary storage (usbhostfs), but only through Linux. Those without the space to spare for an extra Linux partition would do better to create a Live disk instead and set their computer's BIOS boot hierarchy to jump-start the optical drive first.

The UMD mode in contrast works well in both Windows and flavors of Linux, though your PlayStation Portable should be running in Normal UMD mode. You can reset this at the Recovery of CFW 3.71 M33, and usage instructions, along with installation instructions, are included in the detailed documentation bundled with the archive.

Download: Custom Firmware Extender v1.6
Visit: QJ.NET PSP Development Forum

A stream of kudos goes to x1 - Jerry for pointing us toward Cpasjuste's way.