In a number of sources I've come across (all from the Entente side), there has been mention of using machine guns as indirect-fire weapons to supplement an artillery barrage (especially a creeping barrage). This does not seem on the face of it to be an effective use of such weapons, so I was wondering if this was indeed a real tactic, how common it was, and what the intended purpose was- casualties, morale damage, give the machine gunners something to do, etc.
Yes, the British used heavy machine guns for indirect fire. And their use in this role was quite common, for most of the reasons you state. Casualties, harassment, and disruption.
Breaking down why; essentially different calibers of machine guns have different uses. The light Lewis and Hotchkiss guns (two to four men crew) are designed to work with small units of troops; they are man-portable, with feeding designs meant for quick re-loading by people carrying magazines. The medium/heavy Vickers gun (six to eight men) is a water-cooled, not particularly portable refinement on the old Maxim gun. All three of these guns are chambered in .303, the standard British rifle round, simplifying logistics. This is a large, powerful round, quite capable of travelling over the horizon (the 7mm Mauser round has a flatter trajectory and a longer range still).
There are a couple of reasons for indirect fire from the heavy Vickers - tactically speaking, it allowed the Vickers to fire suppressing and harassing rounds at over twice the distance that line-of-sight achieved. The effective range of direct fire - flat trajectory while an enemy could be seen - was roughly 2 kilometers. However indirect fire - pointing the muzzle of the gun upward, firing in an arc to land atop an enemy - could reach back over 4, clear over terrain. While high explosive artillery rounds have their charms, it is also expensive, churns up ground, and is often unavailable.
Operationally, indirect fire was a luxury afforded when the Hotchkiss and Lewis guns appeared on the scene in numbers great enough. The Vickers gun crews and their weapons were able to be removed from the Infantry and grouped into their own Corps. This was designed to mass firepower in more effective ways, and coordinate machine guns across the whole army. Suddenly there were battalions of Vickers which could be employed in this way.
TL:DR Opportunity and equipment.
I don't have an answer unfortunately, but as a question to you, why would you assume that this would be ineffective?