TYPO3 Core

I have a file storage for a readonly filemount.
as it is not possible to set the processed folder outside the root of a file storage¹ I need to insert another local folder to enable a local (and writable) processed folder².
as the files from that filemount are accessed from another server this additional folder is superflous.

[1] all entries of .. inside the path to the processed-folder result in an fatal error:
#1320286857: File ../_processed_ is not valid (".." and "//" is not allowed in path). (More information)

[2] as the filemount is readonly there is no possibility to create the processed folder inside the root directory of the file storage (and also no processed files):

PHP Warning
PHP Warning: rename(/var/www/typo3temp/preview_nFDQil.jpg,/var/www/imgservermount/_processed_/preview_aeg-182232_0_0a048ffd3d.jpg): No such file or directory in /var/www/typo3_src-6.1.10/typo3/sysext/core/Classes/Resource/Driver/LocalDriver.php line 622

which results in errors for each 'preview-image' (as there is no preview image):

exec_INSERTquery
caller     TYPO3\CMS\Core\Database\DatabaseConnection::exec_INSERTquery
ERROR     Column 'originalfilesha1' cannot be null
lastBuiltQuery     INSERT INTO sys_file_processedfile (width,height,checksum,identifier,name,storage,task_type,configuration,original,originalfilesha1,tstamp,crdate) VALUES ('64','64','0a048ffd3d','/_processed_/preview_aeg-182232_0_0a048ffd3d.jpg','preview_aeg-182232_0_0a048ffd3d.jpg','6','Image.Preview','a:0:{}','89881',NULL,'1410248566','1410248566')
debug_backtrace     require(o3project/typo3_src-6.1.10/typo3/sysext/filelist/mod1/index.php),o3project/typo3_src-6.1.10/typo3/mod.php#41 // TYPO3\CMS\Filelist\Controller\FileListController->main#46 // TYPO3\CMS\Filelist\FileList->generateList#290 // TYPO3\CMS\Filelist\FileList->getTable#199 // TYPO3\CMS\Filelist\FileList->formatFileList#353 // TYPO3\CMS\Core\Resource\File->process#622 // TYPO3\CMS\Core\Resource\ResourceStorage->processFile#293 // TYPO3\CMS\Core\Resource\Service\FileProcessingService->processFile#1150 // TYPO3\CMS\Core\Resource\Service\FileProcessingService->process#103 // TYPO3\CMS\Core\Resource\ProcessedFileRepository->add#140 // TYPO3\CMS\Core\Database\DatabaseConnection->exec_INSERTquery#123 // TYPO3\CMS\Core\Database\DatabaseConnection->debug#198