The main idea is this: That sieging forces may send a scout to your site either a few weeks-months ahead of arrival or the planned siege, or that an unfriendly site may send a scout over to try having a look at your defenses.
The scout's preferred skills would be climbing, tactician, ambush, observer, and siege engineer, would arrive on the map in stealth and attempt to avoid line of site or proximity to the site's citizens (unlike kobolds who simply try to get as deep as they can and scamper off as soon as detected).
If dwarves are to be given the ability to directly fight, respect, and avoid fire, perhaps our scout can set an occasional fire as a distraction.
The main intent of the scout is to observe alternate routes and the respective density of dwarves in certain areas of the map, and come up with a siege plan (breach a particular out of the way wall with a constructed siege engine, infantrymen flooding over a wall by climbing a nearby tree and hopping in, taking consideration of drawbridges with mechanisms, trap detection, et cetera.
The idea is that the plan is to be set up beforehand, so the sieging units will be set on performing the plan unless the environment changes enough to invalidate it. This also means that sieging units can bring the proper tools for the job rather than simply giving invading siege engineers free reign over what they can do or leaving it up to random chance (team of spades to breach soil, a large ladder formation for open top fortresses, a large wagon-driven wall slammer for cracking a wall, etc).
If a scout is detected before they finish operations and leave, they will attempt to kill a witness if they are not too threatening, or simply flee off the map like other detected infiltrators. They might be convinced to surrender if surrounded (assuming such a thing is possible when the update for it comes around).

Likewise, player fortress can send expeditionary scouts to sites to infiltrate and return with both "vague" and "specific" reports. The specific reports are simply information acquired that the scout uses minimal personal risk or profile to acquire (no bullying or fighting for it like our normal missions) where as the "vague" reports provide a risk mitigation bonuses to proper missions to the site, for as long as information is relevant.