Tip: Layer 3 backgrounds and tides disappear when you hit a message box, so unless you apply this fix, plan your level around that. For example, you can add a sublevel without layer 3 at the start/midpoint, and include your message there.
It is possible to activate various effects according to the sprite number that Yoshi has in his mouth.
You can also negate the special abilities of Yoshi's color, the color of the shell he has in his mouth. (Even if it is disabled, you can activate special abilities with other sprites.)
For example, you can explode a Bob-Omb in Yoshi's mouth, or eat a fire enemy to spit out flames.
You can also put a specific sprite in Yoshi's mouth to activate special abilities related to the color of the shell.
Also, when the bob-omb explodes, the tweaker settings will be rewritten so that it will no longer be affected by other explosions, cloaks, and invincibility, and will no longer be eaten by Yoshi.
This patch was on the fence as it's rather rough around the edges, but it does do some neat stuff, and its foibles are mostly inoffensive.
First things first - there were three improperly SA-1-hybridized RAM addresses as well as a missing instruction in the carried sprite ability code, that between them created some rather severe bugs. I feel like this should have been noticed given reasonable testing, but at the same time the fix is a meager four lines, so I've corrected them. I also commented out a small hex edit that has no effect as it is skipped completely by a nearby jump.
The patch itself could use some usage instructions. To carry around a custom sprite, you have to first ensure that its "stays in Yoshi's mouth" Tweaker bit is set. You must then separately enter InMouth_Sprite_Check.asm and Spit_Sprite_Check.asm and add a CMP/BEQ pair as shown in the included examples. InMouth_Sprite_Check is not mandatory, but you'll get weird results if the custom sprite has no entry in Spit_Sprite_Check. Both InMouth_Sprite_Check and Spit_Sprite_Check have their own entries for the "continuous fire" effect, and both must be set for a given custom sprite in order for the ability to function properly.
Speaking of continuous fire, I'm really not wild about how it doesn't bother to enforce a Y speed for the spit fireballs - they just use whatever junk happens to be in $173D for the slot that the fireball ends up in. If you wanted the fireballs to have a pseudo-random trajectory, I would much rather see the actual RNG used instead. As-is, depending on what other extended sprites have come and gone, you might end up spitting fireballs that all veer sharply up or down. There was one other curiosity, in that falling just right while licking up a Para-Bomb can allow the player to bounce off the bomb while it's still on Yoshi's tongue. It still ends up in Yoshi's mouth and behaves normally afterwards, though.
Finally, from a design perspective, I personally find it odd how the continuous fire ability and the "bombing" ability (the "run very fast" ability) are custom-sprite only. Don't get me wrong, the extra abilities are nice, it just seems like all the things that can be done are oddly forced into a very specific mold.