The mantlet rules in AW do an excellent job of modelling basic sparabara tactics. Here is a subtle tweak that adds a little more flavor.Years ago there was a running debate in Slingshot between Richard Bodley Scott and Phil Barker re: whether sparabara fought in close (Scott) or loose order (Barker). Presumably the bow(X) designation in DBM is the compromise solution. The following rule accommodates both views with more elegance than DBM (as befits AW).

Sparabara in Loose Order. All sparabara are assumed to be in loose order. When trained sparabara deploy their spara, they go into close order. Trained sparabara remain in close order so long as they successfully defend their spara-wall -- and they do not move -- or they are issued a deploy order to pick the spara up, at which point they immediately revert to loose order.

Deployed Spara. The spara wall has to be deployed in order for it to count as an obstacle and a superior tactical position; otherwise it merely counts as cover plus shield versus missile attacks (like an MW pavise). Only trained units may deploy the spara wall; thus, irregular sparabara never get to deploy the spara wall nor do they ever go into close order. The spara wall is breached and the spara are assumed lost whenever a defending sparabara unit is pushed back or routed, or whenever a successful sparabara unit follows-up or pursues; any movement by the owning unit without first picking up - via a deploy order -- the deployed spara results in the loss of the spara. Note, since the defended spara count as an obstacle, which will disorder the defenders if they move through it, the sparabara are not obligated to follow-up or pursue as per normal AW rules (unless they go frenzied).

Sources

I've relied most heavily on the work of Nick Secunda, but Duncan Head's analysis of OoB's was very helpful, as was Peter Green's discussion (1) of numbers re: Xerxes invasion army in 480.

Finer, S.E.
One of the best reads regarding the Achaemenid Persians is Chapter II.1 in Ancient Monarchies and Empires, the first volume of S.E. Finer's A History of Government (Oxford, 1997). Finer's study details the history of government institutions by recognizing the original contributions of cultures past and present in the creation of aspects of government we now take for granted. The case of the Persians is very interesting because one must answer the question: how did a small aggregation of relatively poor tribal clans from the hills of Iran come to establish the largest empire -- at the time -- ever known? That the Persian empire lasted approximately 200 years is a testament to the effectiveness of Achaemenid rule, a perspective often lost if one narrowly focuses on its military failures, as one is naturally inclined to do reading the mostly Greek source material. Those interested are strongly urged to read Finer's analysis.

Green, Peter. (1) The Greco-Persian Wars, California. 1996 (2) Alexander of Macedon, 1996.
Head, Duncan. The Achaemenid Persian Army, Montvert Publications, Stockport, 1992. Herodotus The Histories.
Olmstead, History the Persian Empire, Chicago UP, 1948.
Sekunda, Nick (1) “The Persians”, in Hackett, General Sir John. Warfare in the Ancient World. Facts on File, West Germany, 1989. (2) The Persian Army 560-330B.C. , Osprey Elite Series #42, London, 1992.
SOA, 35 years of Slingshot
Warry, John. Alexander 334-323 B.C., Osprey Campaign Series #7, London, 1991.

More Achaemenid Persians

Introduction
A Persian Primer
A Great Power Emerges 550 - 500BC
Attack on the West 500 - 400BC
Civil War 400-338BC
The End 338-329BC
Sparabara House Rule