Electrical – put a low-voltage transformer in the attic

I was not able to find any sections in NEC that prohibit switches from being installed in closets, so until somebody can point to a specific section I'd say it's not a problem. There may be adaptations to the code in your local area, so you'll have to check with the local Electrical Inspector to be absolutely sure.

The NEC does, however, have a section on luminaires in clothes closets.

NEC 2011

410.16 Luminaires in Clothes Closets.

(A) Luminaire Types Permitted. Listed luminaires of the following types shall be permitted to be installed in a closet:

(1) A surface-mounted or recessed incandescent luminaire with a completely enclosed lamp

(2) A surface-mounted or recessed fluorescent luminaire

(3) Surface-mounted fluorescent or LED luminaires identified as suitable for installation within the storage area

(B) Luminaire Types Not Permitted. Incandescent luminaires with open or partially enclosed lamps and pendant luminaires or lampholders shall not be permitted.

(C) Location. The minimum clearance between luminaires installed in clothes closets and the nearest point of a storage space shall be as follows:

(1) 300 mm (12 in.) for surface-mounted incandescent or LED luminaires with a completely enclosed light source installed on the wall above the door or on the ceiling

(2) 150 mm (6 in.) for surface-mounted fluorescent luminaires installed on the wall above the door or on the ceiling

(3) 150 mm (6 in.) for recessed incandescent or LED luminaires with a completely enclosed light source installed in the wall or the ceiling

(4) 150 mm (6 in.) for recessed fluorescent luminaires installed in the wall or the ceiling

(5) Surface-mounted fluorescent or LED luminaires shall be permitted to be installed within the storage space where identified for this use.

Regarding your code question: no, you cannot hardwire the transformer.

I believe this falls under NEC 400.7:

400.7 Uses Permitted.

(A) Uses. Flexible cords and cables shall be used only for the following:

1. Pendants
2. Wiring of luminaires
3. Connection of portable luminaires, portable and mobile signs, or appliances
4. Elevator cables
5. Wiring of cranes and hoists
6. Connection of utilization equipment to facilitate frequent interchange
7. Prevention of the transmission of noise or vibration
8. Appliances where the fastening means and mechanical connections are specifically designed to permit ready removal for maintenance and repair, and the appliance is intended or identified for flexible cord connection
9. Connection of moving parts
10. Where specifically permitted elsewhere in this Code

At a stretch, if you could permanently attach the transformer, you could maybe consider this to be under 400.7(A)(8), but then you're still required to have a removable plug:

(B) Attachment Plugs. Where used as permitted in 400.7(A)(3), (A)(6), and (A)(8), each flexible cord shall be equipped with an attachment plug and shall be energized from a receptacle outlet or cord connector body.

Additionally, I think this could be interpreted as violating 400.8:

400.8 Uses Not Permitted. Unless specifically permitted in 400.7, flexible cords and cables shall not be used for the following:

1. As a substitute for the fixed wiring of a structure
2. Where run through holes in walls, structural ceilings, suspended ceilings, dropped ceilings, or floors
3. Where run through doorways, windows, or similar openings
4. Where attached to building surfaces Exception to (4): Flexible cord and cable shall be permitted to be attached to building surfaces in accordance with the provisions of 368.56(B)
5. Where concealed by walls, floors, or ceilings or located above suspended or dropped ceilings
6. Where installed in raceways, except as otherwise permitted in this Code
7. Where subject to physical damage

So I'd say, no, you cannot just hardwire the transformer in any way.

Additionally, you need to ensure the transformer (and all connections) are accessible for servicing. Transformers also get hot and need cooling - failure to have adequate cooling could cause the transformer to prematurely malfunction and/or lead to all the other problems of hot things in contact with flammable materials like your walls.

I'd find some area (such as under the stairs) that is accessible and unfinished, and run a new receptacle there. Then you can plug in the transformer, run the low voltage wires back to the LED strips, and you're good to go.

It's quite common to remotely mount transformers -- eg, it's very common for low-voltage under-cabinet lighting in kitchens that the transformer is located in the basement near the electrical panel or other unfinished (serviceable) area.