ActionWebService and Key/Value Pairs

I’m trying to import data from an existing database that relies on
key/value pairs. I’d like to set up an ActionWebService server for this
data and have that system send a message to me when it receives
something new, but I’m unsure of a way to handle the parameter
specifiers on my end.

Each request will contain a variable-length list of key/value pairs, but
ActionWebService seems as though it expects a structured list of
parameters. As such, I’d have to explicitly expect a list of about 800
parameters from my client, and not only is that ridiculous, but not all
the objects in the database have values for all the keys, so it’d just
end up being empty sets being sent over the wire; hardly efficient.

Am I going about this wrong or is this a limitation of AWS?

Thanks,
Adam