@Dave Blackman I would think that the inability to put your Missouri properties on the umbrella has more to do with the number of properties you have than it does with crossing state lines. You should be able to purchase one umbrella for all of your investment properties (regardless of state lines) and one umbrella policy for your personal insurance needs (home, auto, etc.).
A typical structure is to have a general liability policy specific to each property ranging from $100,000 to $1,000,000. (Increasing these limits is pretty inexpensive) You then have one umbrella policy ($1 million to $10 million limits) that can provide liability coverage for all of your properties in the case that the primary layer is exhausted. It is unlikely that you will ever need this extra layer of coverage but for $500 or so it may be worth the assurance you are protected.
I think it is worth noting three major differences in the MO and CA property insurance markets.
Earthquake: a major fault line runs through St. Louis that has not been active for the last 100 years. However, buildings in Missouri are not built to withstand an earthquake so if one were to ever occur most older properties would be totally leveled. For this reason purchasing earthquake insurance is something I would consider.
Wind/Hail: It rains in St. Louis around once a week and large hail is a regular occurrence. For this reason, your wind/hail deductible is much more important. If your properties have older roofs it is only a matter of time until you have a hail claim.
Agreed Valuation: Most of the rental properties with good cash flow our close to downtown and can be upwards of 100 years old. The cost to replace these homes as they are would likely be much more than the way you would actually rebuild (i.e. brick covering 100% of the home, wider hallways etc. For this reason, I recommend trying to get an agreed value policy which allows you and the insurance company to agree on the value of the home upfront. This strategy allows you to agree on a value that more closely reflects how you would actually rebuild the home without being subject to an under-insurance penalty.