Brent, these blokes might be able to help. http://www.fotechsolutions.com/index.php/products/oil-gas-downstream
We investigated the system to pick up temperature differences downhole along the entire string (it was during production though) but it might be a solution. You would be able to detect the interface between fluids (gas/air/water/mud etc)

1. Wellbore pressure sensors located in the BOP stack work good. These are installed in the offside outlet from a failsafe valve. You'll need a reliable wetmate connector to carry the electronic signal from the lower stack up to the MUX pods on the LMRP. Other option is to install the sensor in the choke or kill line on the LMRP, eliminating need for the wetmate connector and its hydraulic ext/ret functions from the MUX pods.
2. Displacing the choke or kill line with sea water and monitoring the riser differential pressure seen on surface will indicate a drop in fluid level in the riser.
3. Most riser manufactures will make a riser fill valve that is either automatic opening or manual opening. Normally installed within 300-500 feet of surface. Depending on the collapse rating of the riser and contracted water depth for the drilling unit, this would be an essential item to be included every riser run.
4. Something else to consider is the external pressure rating of BOP stack components. Old style Cameron type U BOPs and HC collect connectors have a very low external pressure rating, much lower than the collapse pressure of riser. There are OEM bulletins describing these capacities.

In areas where massive losses result in deepwater.
using SBOP where the riser is now a high pressure casing so collapse can be catered for. 

Using downhole PWD, SBOP, MPD, constant bottom hole pressure drilling or PMCD managed pressure drilling techniques are/can be used to monitor measure and control annular pressures.

SE Asia, Caspian are areas that spring too mind. 

Not too sure what you are wanting to do here.
For the well casing and liners these must be designed for worst case. 

Where you expect PVT variations ( e.g. well test, leaks in tubing, gas migration in a cemented annulus etc), Wellcat or similar high end engineering should be conducted to assure your pipe does not fail.
 
Casing annuli on subsea wells are sealed and tested from above at wellhead with metal to metal seals. Normally no annular measurements are made in sealed annuli. 

In open hole lost circulation, the driller must close in the well at SSBOP quickly to prevent riser evacuation and potential collapse. Drilling contractors can advise procedures used to fill well with seawater or base oil for Wbm and OBM Pressure sensors in the bop on modern rigs can help somewhat. However by measuring volumes pumped to get annulus full, you know the loss zone pressure.  Depending on what you are doing drilling or running casing at the time losses occurred. There are various options to then work up and consider.