So since there isn't a way to stack above 127, how would one go about having an item that can hold certain items in it, separate from the inventory, as sort of a portable chest / pouch?

I don't think that is possible because minecraft max id is 64. I wouldn't mess around with any of Mojang's code because you could damage the game. I have tried it myself but it doesn't work that way. Sorry just not possible.                   

 

 

 

 

 

 

I think

Thanks for the heads up on that, updating the question.

You can make an item like an inventory by reading/writing to its ItemStack.stackTagCompound, the same way as chests and furnaces do with their readFromNBT, writeToNBT methods.

How would I do the max stack of 127 though? Just set the max stack to that and it'll work?

You can do the hard thing and create a BigItemStack class which allows you to stack up to 32767 items, but that will require you to handle all of the data handling and nbt writing, crafting recipe handling, furnace recipes, container code, inventory gui, and other things specifically. It's a lot of code to write for one minor adjustment. However, it can be done.

And then sequituri? Patch every IInventory class to support your new BigItemStack?!?

Sounds like fun

 

I think i have Seen a backpack mod somewhere. Maybe the author is willing to share some code.

This isn't that hard.  I created my own backpacks.  In the tiles you can set max stack size.  You just have to deal with only letting someone picking up 64 rather than you entire stack.

Take a look at how chests handle their inventories. It will pretty much be the same for the item except you will need to do some NBTTagCompound specialization, but it's pretty basic stuff. As for the stack size? Just make more slots.