I don't really see the need for wireless iso/homebrew streaming, except for at home... and the same is for usbhost-ing.
Both don't make it possible to play the games on-the-go, and that is what the PSP is primarily ment for: to take it with you.
Now, I have a 2,5" usb harddrive case with USB-OTG, and an iAudio X5 mp3player with USB-OTG. I take the content for my PSP with me, in ready to go folders, on those drives. I can hook up my PSP to the device, and on the device say 'copy to'. So I can copy a movie or ISO or something from my portable harddrive on my PSP while I'm in the train (without bringing my bulky laptop
What you want sounds like USB-OTG support _in_ the PSP. You can plug any portable USB storage device (harddrive, cardreader, mobilephone) to the PSP, and play the ISO's or movies directly from that device.
1) This would require a whole lot of USB driver code _in_ the homebrew software on the PSP to do this... sounds like a lot of work, and you would
run into compatibility issues with all the different devices out there
2) It wouldn't even be that handy! Think about it, your PSP lasts around 3,5 hours on a full battery while playing? Playing movies it might even run for longer. Now, the most portable USB-OTG devices I've seen don't even last that long! My 2,5" harddisk case lasts around 2,5 hours on it's builtin battery.
Now, copying 1gb on the PSP takes at most 10minutes. But playing directly from the device, the battery would be that in two hours or so. And cardreaders/mobilephones/etcc don't have that much storage space to be useful...
So, using USB-OTG devices to fill your PSP on the go works already, and maybe pretty useful. Playing directly from USB-OTG devices is a near impossible programming task I think and wouldn't be that practical!