On the export side :
DIR_TRANS (ABAP transport directory) only comes into play ‘at end’ of export time – CTS(+) will store the uploaded file there (in location as mentioned by you) but there is no way this can be influenced/configured (by application or customer), and portal(/any exporting application) will not know (nor need to know) that.
In principle at (beginning of) export time application needs to store ‘content to export’ (often on file system, but could in principle also be ‘in memory’), how content is then (expected) to be transferred to ABAP/CTS+ depends on setting of export system:
Parameter WBO_FILE_TRANSFER (see also http://help.sap.com/saphelp_nw73ehp1/helpdata/en/5d/b9fc64dd82414c90c72a8c26a9f17b/content.htm) is defining that –usually if not set then Default ‘Stream’ is used [apart from very early CTS+ versions where Stream was not yet supported/implemented]
Only if this parameter is set and with value ‘Share’ the folder where exporting application stores content is relevant (and has to be made known to CTS+ by setting NON_ABAP_WBO_INBOX to a value where CTS+ system can fetch it, using filename only –must thus be unique, to ensure correct content is fetched…).
In all other cases (WBO_FILE_TRANSFER not set or set to ‘Stream‘, as said ‘default nowadays’) CTS+ expects export application to transfer content anyway as byte stream –from where application takes this is completely unknown to/irrelevant for CTS+ (as said in principle it could even be from ‘main memory’, i.e. not stored at all in file system).
So again: The portal (any other exporting application) does not need (esp. not at export time) access to DIR_TRANS –unless NON_ABAP_WBO_INBOX would point to it (and ‘Share’ is configured), but I think this would be a pretty strange set-up [could in some version also lead to strange effects, like unwanted file deletions during export], and I even doubt that portal could be configured in a way to use this folder (not to speak of probably permission issue for JAVA/portal user to write to ABAP transport directory…).
On the import side :
The DeployWS (Web Service) needs to read file from the ABAP transport directory (which in the case of pure non ABAP systems in STMS, this would need to read the ABAP transport directory of the communication system. This would usually work via DEPLOY_DATA_SHARE. This is simplified if the ABAP communication system and Java/DeployWS run on the same instance – Solman for example.
The ‘last file’ under CTS+ control is the file accessed by DeployWS (either via a share, or via Jco -then stored as local/temporary copy on AS Java where DeployWS runs). With this this file a ‘application-specific deploy adapter’ will be called, locally, i.e. on DeployWS host –like SDM Remote API, XI Deployer, DeployEJB or local script.
If there is need to ‘remote’ file to ‘final target system’ this is not under CTS+ control, but by the ‘native deployers’ that CTS+ calls.