Hello, I figured part of it out.
After looking at the source code of buildcraft (it is public!) I was able to find out the two following facts:
1) There exists a class net.minecraftforge.common.ISidedInventory, which should be extended by any inventories that are to have different sides.
2) There is no easy way to find whether or not an item can fit in an inventory. The way buildcraft does it, and I hope redpower does the same thing too, is it iterates in all Slots in an inventory, and if their contents are null, figures that it can put in some items, else it calls the finction ItemStack.isItemEqual(ItemStack) to determine whether or not it can place some or all of the stack in that slot. The reason why that is important is because if you wish to make an inventory only accept certain items the you can change the default value of your stacks from null to a custom ItemStack class that will 'equal' all items that you want to put in. This can be used to fix an issue that I have been running into with some buildcraft related mods, which allow you to push some items in slots where they don't belong, e.g. you can put an item in the bucket slots of a Hobbyist's Steam Engine (Railcraft) via pneumatic tubes (meybe bc tubes too, unsure).
I am making a mod that will interact a lot with inventories, so please post ANY information you have on the matter, it can't hurt.