Is there any emerging standard notation for the variety of different roll styles the marimba is capable of (alternating between both hands at the same time, rolling 1-4 or reverse, any other techniques that the average composer may not be familiar with)? Alternately, talk about the different styles of rolling available with examples. Is it something that the composer generally leaves up to the player or that they specify? Are they more likely to specify if they are a percussionist moonlighting as composer?
In general, there are three different styles of rolls that composers use. Thankfully there is a fairly standard way of notating these styles. However, it is a safe bet to use a key to inform the performer of what you mean. Let’s show you some examples of what I mean:
(You can click any of these examples to view a larger version)
Performance Note Keys


Traditional (2+2) Rolls:

Most of the time this is what is written. A lot of composers will only write out traditional rolls, leaving it up to the performer’s discretion if they choose to play ripple rolls or not. Some marimbists prefer to almost always play LH + RH, some prefer to almost always ripple roll…
Ripple Rolls:

Again, a lot of composers will choose to not worry about ripple rolls because the performer will oftentimes choose whether they want to ripple roll or not. You often see this notation in works by percussionists, or works that have been edited by a percussionist prior to publication.
One Handed Rolls:

The use of notating the one handed rolls is if one hand will at some point stop rolling and play an articulated line.
Here are a couple other tidbits:

This is an older way of notating normal rolls. I personally don’t like this style, because it confuses me. I prefer to see all of the notes stacked over each other.
As far as specifying the ripple roll direction (1-2-3-4; 4-3-2-1; 1-2-4-3; etc.). I would leave that up to the performer. It would change from performer to performer based on what feels comfortable to them. Most of the time when I play a ripple roll I go up from 1 to 4. This is because I like to play the bass note first to give the chord some resonance; and I like to play the top note last (if it’s a melody) so that it’s easier for me to bring it out.
Hopefully I covered everything in Brian’s question. These are really the only 3 styles I’ve found. However, if any readers have seen any other examples please share!