For this I think you will need to create a customised panel layout. You then call the webservice in your customised layout, via a button or such, and place the result in a mx.controls.Label control (by assigning the result of the webservice to the htmlText property).
There is some help on how to do this and the sample code provided with the LiveCycle SDK is pretty much the source code for all of the guide layouts, panel layouts and controls used in a Form Guide (so, for instance, if you are using a One Column layout, you can then just add your customised Label and Button that calls the webservice to the SampleOneColumn MXML file).
(Sorry if I am way off track with what you are trying to achieve!)
Here is a great video and the Adobe doco on how to customise Form Guides:
Video:
http://www.adobe.com/devnet/livecycle/articles/customizing_formguides.htmlDoco:
http://www.adobe.com/go/learn_lc_fgCustomizing