The issue I am having is it appears many modern remotes don't have a 38kHz carrier frequency for the infrared signal. I would get inconsistent readings using an TSOP 1738. Since the remotes you can buy as a kit are always cheap and ugly, how hard is it to replace the circuitry with an attiny device to generate the correct frequency?
How hard is it to replace the circuitry - Not hard at all.
Here's a circuit for the sender unit capable of 16 unique single key presses.
The uC has three basic jobs to do.
(1) Scan/read the keyboard and determine which key is pressed.
(2) Convert the 'keypress' to a particular code value (see standard codes e.g. RC5) with various start and stop bits
(3) Send out a stream of bits with the 38kHz carrier (internally generated) to the output pin.
You'll find the necessary software in most libraries or other uC projects. From a power saving point of view you should use the key press to wake up the processor.
How hard is it to send IR data using a 38KHZ carrier? Pretty easy. Could you easily strip out the internals of a remote and replace it with your own? Probably just as easy. Have a look at the TVBGone code here:
An ATtiny85 controlling IR LEDs for remote TV control
It turns on/off the PWM generated based on code structures stored in the flash, pretty easy to understand.
See design notes