I received no feedback on my question for tapping the interior fuse box... so I just went out and tested the available ones. I have a US-Spec 2011 Model Year CR-Z built in November 2010. It is equipped with the CVT and is an EX with Nav-system. Your particular fuses may vary depending on vehicle configuration or changes by Honda between model years.
Potential DIY benefits of this include:
1) Adding a relay so direct-to-battery configurations can be created that are switched based on key-position.
2) Adding small (low power) electronics such as heated seats, radar detectors, and backup cameras
3) Adding a USB/IPod charging cable that can provide power regardless if there is a key in the ignition (the normal interior 12v accessory port requires the key to be in the [Acc] position)
DISCLAIMER: This post is for informational purposes only. I am not a Honda technician, and I do not own a Factory Service Manual. I am merely sharing my findings for the forum. I do not take responsibility for any damage/harm that could arise if you attempt any electrical modification based on the information contained below.
Here is a picture of the USA-Spec interior fuse panel (the one by the driver's left knee). You can see that some of mini-fuse spots are available. Below, I explain whether or not you'll get 12v out of it if you tap into the open spots.
4 always on
18 switched ACC or On
19 switched ACC or On
40 always on
41 always on
46 always on
18 and 19 are potentially very useful since they are activated only when the key is in the Accessory or On position.
I didn't test the "Backup" one since I really don't understand why Honda's engineers uniquely labeled it with a special box around it. I just assume it'd be a bad idea to mess with it. I also didn't test #61 and #62 because I could not locate the slots.
I would wager that the "should have been in the USA-spec CRZ" heated seats are usually routed through #18 and #19. This feature is unavailable for USA Market vehicles, so the fuses are physically missing and there is no mention of the circuit in the owners manual or on the fuse-label-sticker.
What is potentially problematic is that the "tap a fuse" (aka "add-a-circuit") kits you get at auto parts stores tend to cover up some of the surrounding fuses. This means it's basically impossible to tap into 18 and 19 simultaneously with two separate fuse taps.
Here is the typical mini-fuse tap. The 90-degree bend will obscure the fuse directly above it - which means #19 will cover #18.
As a workaround, I have decided to tap into: #5 Back Light
I'm assuming #5 is the white light you see when you're in reverse. I couldn't verify this though. I believe I'm in reverse roughly 0.0001% of the time I'm driving, so this circuit won't have load on it during normal circumstances. The unique part of this circuit is that it is available only when the Key is in the [On] Position. That means the item you hook up to it will not be available if the Key in the [Acc] Position.
Also, be aware my vehicle is equipped with the CVT transmission. Just for fun I checked #43 (MG Clutch) and found that circuit provides 12v all the time.
I understand that the "tap a fuse" kits are supposed to run independently of the parent circuit. That is, if you add your 10-amp "tapped accessory" off of the 20 amp regular circuit, that the accessory you added should not impede the original item. Basically everything is run parallel coming off the 12v system in your car anyway... so your new tapped circuit is technically the same (isolated with its own fuse)
Where I get a bit queasy is the fact you're basically shoving a shoddy piece of plastic/metal on an existing line, and I don't want to risk anything to the native feature that is now routed through the fuse tap. Automakers test their equipment and vehicles in some pretty crazy environments. I don't think the "Made in China" company does the same rigorous testing.