Good point on the digging / search radius - I thought it worked that way, but since you had originally posted otherwise, I thought I must have been wrong. As it turns out it affects both, which is actually what I thought!
For rage, if you'd rather just reduce the rate at which rage falls to zero, then you're probably going to want to modify the counter, sp_rage_map. This counter controls the rate at which rage falls. This counter is located in SPECIAL_PARAMS.TXT, by the way...
Next, is how do you want to modify this counter? You can add it to an item if you wish to have it equipped all the time, but a better way may be to add it to a skill. That way, once you select that skill in your skill tree, you'll have it from this point forward (as an aside, you can also implement it virtually anywhere, but let's stick with these two ways for now).
I don't really see much difference between starting at full rage or letting if fall very slowly after combat, but if that is really what you'd rather do then if you go the skills route, you'll need to edit at least SKILLS.LUA. For example, look at the function "skill_wizdom" in SKILLS.LUA. You will see how counters are modified (essentially permanently) with a function - it is very similar to changing mana or rage, but instead you'd be changing sp_rage_map.
If you go to "wizdom" in SKILLS.TXT, you will note that it has a script= line and you will see that for wizdom it is script=skill_wizdom (which matches the function in SKILLS.LUA). So you can either add code to a skill that already has a function call (just like wizdom above) or add a script= line to another skill such that when you choose it, it will increase sp_rage_map. Supposedly sp_rage_map is a decrease in decay rate by that value so if you set it to 100 then it should fall twice as slowly.
If you want to go the item route and just add the bonus to the item then go to ITEMS.TXT and search on sp_rage_map (for example: blackthorn_crown has sp_rage_map=count,50 for a 50% slower rage decrease). This is an easier way to do the rage thing, but you'd have to have the item equipped at all times for the behavior to work. Of course, if you do it this way, it is easy to disable it as well by unequipping the item.
So your choice - you've got a couple of options there.
Good luck!
/C\/C\