"The third stage, the indictio belli, took place on the thirty-third day when a messenger formally declared war and then hurled a magical spear, which had been dipped in blood or pointed with iron, into the land of the enemy to counteract his power. . . . As the Romans extended their power beyond the Latin communities, certain parts of the fetial ceremony became difficult to enact. . . enemy land into which the spear was to be thrown was often either remote or inaccessible. The Romans surmounted the latter difficutly by adopting a curious legal fiction. They had a prisoner of war purchase a piece of land in the Circus Flaminius district and then declared that land hostile territory. The fetials [diplomatic priests] could then cast the spear into it."
This curious example of territoriality is hardly unique in the area of diplomatic law. For instance, there is a longstanding legal fiction that embassies are part of the sovereign soil of their home countries. This fiction serves to limit the power of the surrounding country to enter or to enforce their local laws. Thus, the US embassy in Saudi Arabia can serve alcohol, allow women to work with men, etc. In both cases, the fiction of sovereignty is granted essentially for diplomatic convenience. In the case of the fetial ceremony, it saves the time and effort of travel. In the case of modern embassies, it provides diplomats with a "safe" environment from which they can conduct diplomacy without fear of intimidation or undue discomfort from the host state.
Both examples are pretty strange; the guest countries do not actually weild any power in that territory except by the grace of the host country. But the fetial ceremony is particularly odd because it conveys sovereign status onto soil that belongs only to a citizen of the foreign state. The citizen is made a symbolic representative of the state to which he belongs. His property is foreign soil.
My guess is that the fiction was only valid for this purpose. But what if it weren't? What if a foreign sovereign's property were actually the sovereign soil of his host country--would that make a difference in other areas of law? Let's examine it in the Roman context:
In-home protections against the government were pretty strong, so it wouldn't particularly matter whether a foreigner's home were on foreign soil--representatives of the state (soldiers, presumably) wouldn't be permitted to enter even if it were Rome's sovereign territory. However, in the event that someone broke this rule, it would presumably be an act of war. This would effectively be an extension of the idea that persecuting the citizens of a country can be a justification for going to war (kind of like the so-called "passive personality" principle of jurisdiction). In this case, it would protect the real property of the foreign citizen in addition to his bodily integrity.
Other possible implications: taxation might be affected if the area bore crops. Presumably only the sovereign country could tax the land (although the host country could, I guess, enact some sort of tax on crops traveling off the property). Another area might be emminent domain, which (if such a concept existed at the time) would not apply to territory owned by foreigners. And crimes committed within the house would be under the jurisdiction of the sovereign state. These rules are essentially the same as the current rules for treatment of embassy property.
You're an ancient Roman lawyer representing a foreign citizen. What other implications might this symbolic doctrine have?