I think have to do it with the SharePoint Webservices. Find the list in the specified web and update it.
I recommend the SharePoint 2010 Client Object Model to do this:
//Use SP2010 Client Object Model to update the list
ClientContext SPContext = new ClientContext(wssUrl);
//Get list by name
string listname = "issues";
var query = SPContext.LoadQuery(SPContext.Web.Lists.Where(l => l.Title == listname));
SPContext.ExecuteQuery();
List myIssueList = query.FirstOrDefault();
//Add an item
ListItemCreationInformation nItem = new ListItemCreationInformation();
nItem.LeafName = "Blubb..";
myIssueList.AddItem(nItem);
SPContext.ExecuteQuery();
If you wan't to get the Workspace Url via the Project Id, you can do this by WSSInterop Webservice of the Project Server:
//Use WssInterop Webservice to get the Workspace URL
WssInteropSoapClient wssinteropSvc = new WssInteropSoapClient();
Guid prjGuid = new Guid("30937680-39FA-4685-A087-90C73376B2BE");
ProjectWSSInfoDataSet wssData = wssinteropSvc.ReadWssData(prjGuid);
string wssUrl = wssData.ProjWssInfo[0].PROJECT_WORKSPACE_URL;
I don't know if the code will compile, but it should work like this.
Regards Florian