Say you have Replaygain set to "Off" in the playback settings. Now start playing a fairly loud track. Say the volume's at -25 dB and it's playing at a normal volume level. If I turn the volume up to -5 dB it would be blaring, but not clipping.
Actually, that probably would be clipping if the source is an mp3.
Theres two kinds of clipping on an mp3 player. The first is the one you're thinking of, analog clipping due to the amplifier being driven above 0dB. The second is digital clipping, due to the level at the DAC being driven above 1.0. Replaygain combined with not setting the volume above 0dB prevents both types, but only as we've implemented it.
If it had a replaygain value of +20.00 dB and replay gain was on, it would clip all over the place?
If a file actually has a +20dB RG tag, it won't clip at +20 dB generally, but if you want to be sure, you can enable the "prevent clipping" option in rockbox. This will prevent clipping even on tracks you've edited the tags.
How can my method cause clipping if its turning up the volume and disabling all gain handling?
Because you're effectively disabling replaygain's clipping protection. If you play a normal peak normalized mp3 track without replaygain (or with your method) you'll certainly get clipping unless you compromise and lower the gain through a preamp because quantization error pushes the level above 1.0. But a preamp sucks since you'll lose volume needlessly. Replaygain fixes that by allowing you to have the maximum possible volume that does not clip.
Here's why I'm doing this: I have some podcasts that are pretty quiet compared to other files. I manually put my own ID3 tag on there with a replaygain_track_gain of +10.00 dB. I just want my Rockbox player to play it 10 dB (or 10 volume clicks) louder without it clipping.
You can already do this without modifying rockbox or your replaygain tags:
1) Undo any manual changes to your replaygain tags.
2) Turn the replaygain preamp all the way up to +12 dB.*
3) Select "prevent clipping" under replaygain option in rockbox.
4) Set the volume to 0dB.
This will give you the loudest possible undistorted output your player is physically capable of for a given track. If you can tolerate some clipping, you can push the volume up to +1 or 2dB. If thats still not loud enough you can enable dynamic range compression in rockbox which will squeeze the dynamics in a track to make it sound "louder". Beyond that you'll need an amp or better headphones.
*If you just want to do it for one track, editing the tags is ok as well.