The compose function is not exactly the same as a dead key.
Strictly a compose is stopping spacing and printing the following characters in the same place. So even if your font have no è it would be created from e and `. They are still two characters in the same space.
A deadkey is somthing else: you type the deadkey "`" and see nothing. but a following "e" will be replaced by an unicode char "è" which is one character (and not two in the same space).
A deadkey will only work with combinations which will result in unicode chars, while compose work with all keycombinations (in theory, it depends of the OS and aplication)
There are combinations of both methods (a compose with a valid unicode result automaticly converted in Unicode), so you compose C= results in a real € char and not simply printed C and = in the same place.
Composing the alphabetic char gets first, then the modification follows. Using Deadkey the modificatin (deadkey) gets first, then the alphabetic char.
For example in windows:
deadkey "`" and "e" (Unicode U+0060 and U+0065) will result in "è" (unicode character 00E8)
e +0300 <\ALT> will be get the same optic, but two chars (Unicode U+0065 and U+0300)
Of course real unicode is the more stringent method. But compose is more flexible:
" and "w" (Unicode U+0060 and U+0077) will result in "w" (no unicode character availible), you dont get a composition
w +0300 <\ALT> will be get the the ` over the w as composition an not unicode char, so it works (if its usefull is another question)
A keyboard-layout with deadkeys can be programmed with free microsoft keyboard layout converter, so you can remap deadkey functions to layers (for example, the AltGr layer is allmost empty, you can map deadkeys there), a goot help for the needed definitions is the greek keyboard layout.
Compose keys can also be mapped, but dont click deadkey in the definition but simply type the unicode of the valid composekeys (beginning at U+0300)
The hard thing will be compose and deadkeys on a WindowsPC with restricted user rights (in office), where you can´t make driver changes and start own programms.
For this you can use an programmable keyboard (for example POS-keyboards) or an microcontroller converter (Soarer-Converter with teensy ++2.0). There you can put the composable chars in Unicode in extra layers.
A way to make real deadkeys in hardware is not easy done - you have to programm your own adapter, a nobody has done it now (but Soarer is thinking about developing his converter for it)