You can use the Microsoft.TeamFoundation.Build.Client
and related assemblies to update the process parameters via PowerShell or C#. The one tricky part is the parameters are stored in XML so you have to deserialize, make your changes, then serialize again to set them.
This likely won't run without some tweaking but here are some snippets from one of my scripts that will help:
[void][Reflection.Assembly]::LoadWithPartialName('Microsoft.TeamFoundation.Build.Client')
[void][Reflection.Assembly]::LoadWithPartialName('Microsoft.TeamFoundation.Build.Workflow')
$projectCollection = [Microsoft.TeamFoundation.Client.TfsTeamProjectCollectionFactory]::GetTeamProjectCollection($TFSUri)
$buildServer = $projectCollection.GetService([Microsoft.TeamFoundation.Build.Client.IBuildServer]) $buildServer = GetBuildServer -TFSUri $TFSUri
$buildDefinition = $buildServer.GetBuildDefinition($TeamProjectName, $BuildName);
...
$parameters = [Microsoft.TeamFoundation.Build.Workflow.WorkflowHelpers]::DeserializeProcessParameters($buildDefinition.ProcessParameters)
$msBuildArguments = $parameters.Get_Item("MSBuildArguments")
$msBuildArguments = "$msBuildArguments /p:ImportParametersFilesOverride=true"
$parameters.Set_Item("MSBuildArguments", $msBuildArguments)
$parameters.Add("GetVersion", "c$TFSChangeSetNumber")
$buildRequest.ProcessParameters = [Microsoft.TeamFoundation.Build.Workflow.WorkflowHelpers]::SerializeProcessParameters($parameters)