I'm not really sure what you mean when you say "that Scribus 1.4.6 doesn't support pixels". If you mean that Scribus doesn't allow pixels as a unit of measure, that is because Scribus was created to produce PDFs that are destined for printing so it lets you use units of measure that are used in the printing industry.
That shouldn't really be an issue though as the conversion between pixels and points is quite easy.
If you are making titles/slides for a 720p video - 1280x720px - then you can create a document at 1280x720 points and then change the resolution in the export function accordingly (usually 72 dpi). As long as your document has the same aspect-ratio as the output you want then you can simply re-scale it as necessary on export. Just make sure that the images used are at least a good enough resolution.
In other words, a 1280x720px image, when put into an image frame that covers a 1280x720pt page, exported at 72dpi should give you an image that is identical to the original. However, the exported image quality will depend on the import image type (JPG/PNG/etc.) and export image type and quality settings. An imported PNG on a page exported as PNG should result in exactly the same image (theoretically at least).
Does that make sense? Or have I misunderstood the issue?