All sorted, excellent.

I solved the conflict (in my "anonymous" comment above) by performing a two-stage update: dnf upgrade --best --allowerasing forced the uninstall of qt-creator-4.5.0-1.fc27 in order to update everything else. Then a manual dnf install qt-creator downgraded qbs from 1.10.0 to 1.9.0 to successfully install qt-creator-4.5.0-2.fc27. Odd that dnf couldn't automatically solve this on its own, though.

Success! \o/

Before:

% sudo dnf install gstreamer1-plugins-entrans gst-transcoder
Last metadata expiration check: 1:53:10 ago on Wed 12 Jul 2017 02:33:33 AM EDT.
Dependencies resolved.
================================================================================
 Package                        Arch       Version             Repository  Size
================================================================================
Installing:
 gst-transcoder                 x86_64     1.12.0-1.fc26       fedora      53 k
 gstreamer1-plugins-entrans     x86_64     1.0.3-2.fc26        fedora     100 k

Transaction Summary
================================================================================
Install  2 Packages

Total download size: 153 k
Installed size: 406 k
Is this ok [y/N]: y
Downloading Packages:
(1/2): gst-transcoder-1.12.0-1.fc26.x86_64.rpm   83 kB/s |  53 kB     00:00    
(2/2): gstreamer1-plugins-entrans-1.0.3-2.fc26. 126 kB/s | 100 kB     00:00    
--------------------------------------------------------------------------------
Total                                           116 kB/s | 153 kB     00:01     
Running transaction check
Transaction check succeeded.
Running transaction test
The downloaded packages were saved in cache until the next successful transaction.
You can remove cached packages by executing 'dnf clean packages'.
Error: Transaction check error:
  file /usr/lib64/gstreamer-1.0/libgsttranscode.so conflicts between attempted installs of gstreamer1-plugins-entrans-1.0.3-2.fc26.x86_64 and gst-transcoder-1.12.0-1.fc26.x86_64

Error Summary
-------------

After:

% sudo dnf --enablerepo=updates-testing install gstreamer1-plugins-entrans gst-transcoder
Fedora 26 - x86_64 - Test Updates               1.0 MB/s |  25 MB     00:25    
Last metadata expiration check: 0:00:16 ago on Wed 12 Jul 2017 04:27:34 AM EDT.
Dependencies resolved.
================================================================================
 Package                      Arch     Version          Repository         Size
================================================================================
Installing:
 gst-transcoder               x86_64   1.12.0-1.fc26    updates-testing    53 k
 gstreamer1-plugins-entrans   x86_64   1.2.1-1.fc26     updates-testing   108 k

Transaction Summary
================================================================================
Install  2 Packages

Total download size: 161 k
Installed size: 434 k
Is this ok [y/N]: y
Downloading Packages:
(1/2): gst-transcoder-1.12.0-1.fc26.x86_64.rpm  114 kB/s |  53 kB     00:00    
(2/2): gstreamer1-plugins-entrans-1.2.1-1.fc26. 200 kB/s | 108 kB     00:00    
--------------------------------------------------------------------------------
Total                                           205 kB/s | 161 kB     00:00     
Running transaction check
Transaction check succeeded.
Running transaction test
Transaction test succeeded.
Running transaction
  Preparing        :                                                        1/1 
  Installing       : gst-transcoder-1.12.0-1.fc26.x86_64                    1/2 
  Running scriptlet: gst-transcoder-1.12.0-1.fc26.x86_64                    1/2 
  Installing       : gstreamer1-plugins-entrans-1.2.1-1.fc26.x86_64         2/2 
  Verifying        : gstreamer1-plugins-entrans-1.2.1-1.fc26.x86_64         1/2 
  Verifying        : gst-transcoder-1.12.0-1.fc26.x86_64                    2/2 

Installed:
  gst-transcoder.x86_64 1.12.0-1.fc26                                           
  gstreamer1-plugins-entrans.x86_64 1.2.1-1.fc26                                

Complete!

All working as expected, all fixes verified.

Update functions, but still exhibits bug#1371762 — recoll spontaneously crashes after a short time.