I was recently asked by a customer if there was a way to get custom properties to be displayed in a message on EAS Devices. The short answer is no.  

First, a bit about ActiveSync...  Exchange Server ActiveSync (EAS) is a low-level binary protocol which is heavily licensed. All commercialized development needs to have an EAS developer license and a Premier support agreement with Microsoft.  Every device maker has their own code for doing EAS calls and rending information in their device’s mail client.  EAS knows nothing about client UIs – it only covers the syncing of data between Exchange and a device.  There is no EAS API – customers write all of the own code.  The only exception to this is for Windows Phone Devices, which use the Microsoft Windows Phone OS and mail client.  EAS code is tedious to write and involves licensing.  Usually only device makers use ActiveSync for client applications. 

Since the start of EAS there has never been the ability to work with user created properties (custom properties).  EAS uses a fixed schema and is only expanded through a schema revision which results in an up-rev of the EAS version.  Such schema changes do occur in major releases or service pack releases of Exchange and even then rarely. 

OWA custom forms are only for OWA.  Keep in mind that OWA is its own application and is not an API.  Further custom OWA forms are not used by Exchange APIs.

In order for the custom properties to be displayed on a device, a different API needs to be used – in this case EWS is a good fit.  Further, an application would need to be created to be published to the device which would display the content in the way you want it to be and would of course use EWS.  If all of the fields you need are already in the EAS schema then you could instead try discussing this issue with every device maker to see if they could accommodate the displaying of data you are wishing.

There are other work-arounds which don’t involve custom forms or custom fields. You could include a link at the top of the message which takes the user to where they can get more information.  Also, you could look at adding more information to existing fields which are displayed on a client – such as the subject line, location, etc.  You would need to look at what each client presents in its UI and be creative on where you put such information.  As an alternative to all of this, you could use an OWA custom form and access it through the phone's browser when logged into OWA. These types of approaches would likely be the quickest way to move forward.

This has the schema fields:

[MS-ASWBXML]: Exchange ActiveSync: WAP Binary XML (WBXML) Algorithm
http://msdn.microsoft.com/en-us/library/dd299442(v=exchg.80).aspx  

Shows layout of EAS protocol documentation:

2.2.4 Exchange ActiveSync Protocols
http://msdn.microsoft.com/en-us/library/ee177929(v=exchg.80).aspx  

If you want to know more about ActiveSync development then please refer to the following:

New to Exchange ActiveSync Development?
http://blogs.msdn.com/b/webdav_101/archive/2011/09/28/new-to-exchange-activesync-development.aspx