DARPA suggests that oscillating chemical systems could form time-repetitive sequences of characters, or that a chemical shutter could generate a modulated optical signal from one static chemical function. However, any innovative approach that meets the goals of the proposal will be considered.

DARPA envisions the disposable transmitter having a "weight and form factor ranging from a sheet of paper to a piece of string, possibly with an adhesive backing for deployment on all surfaces."

The "substrate" or transmitter is thus a graffiti tag, but not like the ones that you see in alleys and on trains. DARPA prefers that the message be transmitted invisibly, using near infrared (NIR) or short-wave infrared (SWIR).

Science fiction writers could make several suggestions here. For one thing, you will want a strongly adhesive backing that would resist any efforts to remove it. In other words, a smart tag as described in All Tomorrow's Parties, a 1999 novel by William Gibson:

Someone had once come up with a smart tag, a sort of decal they'd somehow adhered to the [smart] wall, although [they] had not been able to figure out how they'd done it without being seen. Maybe they'd shot it from a distance.
(Read more about the smart tag)

Smart tags were created in response to smart material, a special finish that would "eat" graffiti, which would be the expected enemy response to DARPA ChemComm tags:

The gang kids would come and tag it; twenty minutes later these flat, dark, vaguely crab-like patches of dark blue would come gliding out around the corner… They seemed to be embedded, a few millimeters down into the surface, which was a sort of non-glossy gel-coat affair, but able to move around under there.
(Read more about smart material)

DARPA also does not specify what the transmitter, ideally a "sheet of paper," should look like. If it covers a surface that is visible to a UAV, a person might also be able to see it. The sheet should therefore look like it belonged there.

One approach would be to disguise it as anti-American graffiti or posters of local political figures. Graffiti in Baghdad was once punishable by death; now it is commonplace.

However, in villages, Iraqi army soldiers and Multinational Division–Baghdad patrols often paint over graffiti whenever they see it. You don't want your own troops painting over your expensive DARPA-produced ChemComm tags.

(Child paints over graffiti)

The other obvious thing to do is to make the tags invisible. They could be made of clear, non-reflective plastic, or the tags could actually be invisible. See Invisibility Cloaks Seen As Possible With Metamaterials and Invisibility Possible With Superlenses for details on progress in this area.

As with any DARPA proposal, the technologies are years away from being usable. However, if DARPA wants to get started right away with a quick, homebrew technology that already works, they should try making some LED Throwies.

(LED Throwie)

An LED Throwie is basically a light-emitting diode taped to a small circular battery, in turn taped to a small circular magnet. Change the standard instructions by using an infrared LED, then add a little IC that accepts your 60 character code and transmits it in the DARPA-approved manner, and presto! your chemical communications gear is ready and you can start tagging.

(LED Throwie enthusiasts tagging a building)

I also wonder whether or not DARPA is thinking far enough out of the box on this one. For example, why not have this robot painter actually create a composition on the roof or the wall, using chemical paints that provide the necessary signalling features?