What I don’t understand is why nobody else has come up with this and posted it.
MythTV has been both a bane and a joy over the years, the main bane being getting the remote to work properly. The most annoying and recurring symptom is that most of the keys work, but for some reason, the most important, the “OK” key never does.
The latest kernel now has ir keymaps built in, unfortunately this means that my carefully crafted LIRC configuration for my Hauppuage remote no longer works, and the default keymap settings are as useless as ever, Basically, the 1-9 keys, volume controls, up, down, left and right work, but nothing else.
There must be some well hidden documentation about this somewhere, but after much use of Google and reading webpages, I came to the conclusion that to get the best results without using LIRC as a “bridge” I would need to configure my remote as a keyboard.

To do this, I basically ignored all of the internet posts and guides, apart from the ones that gave me the clues to where the keymap files were kept.
First I installed the ir-keytable package from the repositories.
This also installs a selection of ir keymaps in /lib/udev/rc_keymaps

Using the ir-keytable, command,  you can find out which drivers are in use and which keymaps they use.

on my system, it returns:

Found /sys/class/rc/rc1/ (/dev/input/event7) with:
Driver (null), table rc-dib0700-rc5
Supported protocols: NEC RC-5 RC-6
Enabled protocols: RC-5

I started by looking at the relevant keymap (dib0700_rc5) in /lib/udev/rc_keymaps, and tested the remote to see what buttons actually worked.
Then I ran ir-keytable -t > ~/devtest.txt, pressed every key on the remote in sequence from left to right descending in rows. This gave me a text file with the scan codes and their key attributes.
I then saved a copy of the dib0700_rc5 keymap, and edited it.

The first entry that I tried changing was KEY_OK.

I changed this to KEY_ENTER, rebooted, and lo! The OK key worked! I could now select items in the menus!
So basically, I edited the dib0700_rc5 keymap so that every key corresponds with a key on the keyboard, then going through the key setup in MythTV’s frontend creating links where necessary.

Unfortunately, there are a lot of keys already mapped to functions in MythTV, so this last bit was a little hit and miss, as some symbols or combinations such as CTRL+P don’t seem to work, but finally I can use every key on my Hauppuage remote. I have an MCE remote as well, but I haven’t bothered to do that one yet.
My Keymap is here

Yes I know that using devinput with LIRC as a bridge is more elegant for applications that support LIRC, but I only use a remote with MythTV, and the LIRC solution seems just too convoluted and unreliable to be worthwhile, too many links in the chain.

Note: you need to cold boot, i.e.shut down the PC and then restart it, for the new keymap to be read. I found that the remote would not work after a warm reboot, not sure why.