I confirmed that the latest d2x beta's are available and accessible from ModMii's download page 4 when typing "Beta", same goes for the advanced downloads page
I'm re-reading your message and I think I may have originally misinterpreted it. You can create any custom cIOS you want from ModMii Classic's advanced downloads page. Type "Beta" to change the version of d2x, then select which IOS or cIOS you want to modify. You can change slot numbers and version numbers for all. If you're modifying a standard IOS you also have the option of adding some patches. Then when you're all done if you want you can save the download queue for later and even share it with other ModMii users so you can basically send them a specific\custom cIOS of your design by just sending them a queue and no actual files from any 3rd parties have crossed hands