In direct answer to your question... no, there's probably nothing you can try before you invest in a head gasket.
There are two possibilities here. No, there are three.
First is that your motor oil is entering the water jacket through a breach in the head gasket - the motor oil is under higher pressure than the coolant should ever be, so it'd pressurize the coolant. That, though, would also give your coolant an obvious "milkshake" appearance & consistency.
Second is that your fuel/air mix and exhaust gases are entering the water jacket through a breach in the head gasket. During the compression stroke, and especially during the power stroke, the fuel/air and exhaust are under VERY high pressure, far more than oil pressure. This sort of failure doesn't necessarily alter the appearance & consistency of the coolant, but may cause a very dramatic overpressure condition within the cooling system.
Third is that fuel/air and exhaust are entering the water jacket through a crack in the head, with results nearly identical to head gasket failure. Cracked heads are much less common than failed head gaskets, though.
Cooling system leak sealers will have no effect in any of these cases, short-term or long-term. It may work to stop minor external leakages from the cooling system to the outdoors, but your leak is INTO the cooling system.
If you drive the car any further before getting the problem fixed, you should count on it getting worse - quickly. Even if the leak doesn't get worse fast (which it will), it WILL blow your coolant out, and your car will overheat, and the highest point (the head) will be coolant-starved and suffer from it. Warpage may occur as early as TOMORROW.
If your head is allowed to warp, the car'll never run right again. A machine shop may be able to plane the head, but if they do... every combustion chamber will thereafter be a different size so you'll have different compression (and different power) on every cylinder.