Здесь указаны все поля, которые более, менее можно как-то применить в кастомных полях для материалов, пользователей и контактах.
Укажите в настройках плагина универсального поля путь и при выборе поля JForm у вас станут доступные поля указанные после этого пути.
administrator/components/com_actionlogs/models/fields
extension
logcreator
logsdaterange
logtype
plugininfo
administrator/components/com_associations/models/fields
itemlanguage
itemtype
modalassociation
administrator/components/com_categories/models/fields
categoryedit
categoryparent
modal_category
administrator/components/com_contact/models/fields
modal_contact
voteradio
administrator/components/com_content/models/fields
modal_article
administrator/components/com_fields/models/fields
fieldcontexts
fieldgroups
fieldlayout
section
type
administrator/components/com_finder/models/fields
branches
contentmap
contenttypes
directories
searchfilter
administrator/components/com_menus/models/fields
componentscategory
menuitembytype
menuordering
menuparent
menupreset
menutype
modal_menu
administrator/components/com_modules/models/fields
modulesmodule
modulesposition
administrator/components/com_newsfeeds/models/fields
newsfeeds
modal_newsfeed
administrator/components/com_plugins/models/fields
pluginelement
pluginordering
plugintype
administrator/components/com_redirect/models/fields
redirect
administrator/components/com_templates/models/fields
templatelocation
templatename
administrator/components/com_users/models/fields
groupparent
levels
libraries/joomla/form/fields (указывать путь не надо в настройках, в плагине поля уже сразу грузится этот путь)
accesslevel
aliastag
cachehandler
calendar
checkbox
checkboxes
color
combo
components
databaseconnection
email
file
filelist
folderlist
groupedlist
hidden
imagelist
integer
language
list
meter
note
number
password
plugins
predefinedlist
radio
range
repeatable
rules
sessionhandler
spacer
sql
subform
tel
text
textarea
timezone
url
usergroup