Thanks mjjzf and w_whalley for stimulating my thought processes. Here is the final script, I hope someone finds it useful:
echo There are no guarantees on its actions.
echo For example it needs a limit on the number of files it opens.
for fn in *; do
if [ "$file" = "$project" ] ; then
if [ "$ext" = "odt" -o "$ext" = "ods" ] ; then
libreoffice "$fn" &
Check that all the files you want associated with the project and opened at the same time start with the same name and are .ods or .odt ones.
Save the script into the directory with the same name, give it the executable property.
Double-click on the script to open all the files but no others, example:
club files spreadsheet 1.ods
club files spreadsheet 2.ods
club files document 1.odt
club files document 2.odt