There are no restrictions on B-RNAV because B-RNAV was not designed for use in terminal areas - B-RNAV STARS should be simple with no more than 4 waypoints per 100 track miles and should always be well above MSA. So you can probably get away with manual entry.
However it would be unwise to try and manually build a SID or a STAR for two reasons:
1. There is the question of fly-over and fly-by waypoints, but more importantly, there is the question of path terminators - these are the codes that tell the FMS how to fly each leg - course to altitude, direct to fix, track to fix, radius to fix, course to intercept etc etc - I don't think that any FMS provides the pilot with the means of inserting these codes. In the more complex SIDs the procedure design is based upon the use of particular path terminator codes for particular legs so you wouldn't want to get it wrong.
2. There are still a large number of RNAV SIDs and STARs published by some States in Europe (and perhaps elsewhere) where it is not clear what approval is required. Some of these look a little too complex or close to terrain to be BRNAV even though there is no clear statement of a P-RNAV requirement.